Unit 2 · Supply, Demand and the Market

Maya kept a notebook all summer. On a mild day she tried a different price each hour and counted cups. At 50 cents she sold about 60 cups an hour. At $1 she sold 40. At $1.50 she sold 25, at $2 she sold 15, and at $2.50 she sold only 8. Written as a list of prices and the amounts people bought at each one, this is a demand schedule. Every row answers the same question: at this price, how many cups do buyers want?

The pattern in her notebook is the law of demand. When the price of something goes up and nothing else changes, people buy less of it. When the price goes down, they buy more. Economists say price and quantity demanded move in opposite directions. Quantity demanded is the specific amount buyers want at one specific price. At $1 the quantity demanded was 40 cups. At $2 it was 15 cups.

Notice the phrase "nothing else changes." Economists call this holding other things equal. Maya’s table only makes sense if the weather, her neighbors’ incomes, and the price of soda stayed the same while she changed her price. If the temperature jumped 20 degrees between the $1 hour and the $2 hour, she could not tell what the price did on its own. Real economists face this problem every day and design their studies to get around it.

Demand is the whole schedule, the entire relationship between price and the amount people want. Quantity demanded is just one row of it. Students mix these up constantly, so it helps to use the words carefully. If someone says "demand for lemonade is 40 cups," ask "at what price?" Demand is the full list. Quantity demanded is one number from that list.

Why do fewer people buy lemonade at $2 than at $1? The first reason is the substitution effect. When lemonade costs $2, the water fountain at the park and the 75-cent soda at the gas station look better by comparison. Some buyers switch. They did not stop being thirsty. They found a cheaper way to solve the same problem. Every good has substitutes, even if they are not perfect ones.

The second reason is the income effect. A kid with $5 for the afternoon can buy five cups at $1 but only two and a half cups at $2. The price rise did not change the number of dollars in her pocket, but it did shrink what those dollars can buy. She feels poorer at the lemonade stand, so she buys less. For a small purchase this effect is tiny. For rent or gas it can be large.

The third reason is diminishing marginal utility. Utility is the economist’s word for satisfaction. Marginal means the extra amount from one more unit. The first cup of lemonade on a hot day is wonderful. The second is nice. The third is fine. By the fourth you are mostly full. Since each extra cup is worth less to you, you only buy that fourth cup if the price is low. Lower prices pull in the cups that are worth less to buyers.

Put these three reasons together and the demand schedule always slopes the same way: high price, small quantity; low price, large quantity. A common mistake is to think the law of demand is about what one person feels. It is about what happens across many buyers. Even if you personally would pay $5 for lemonade, the crowd as a whole buys fewer cups at $5 than at $1.

Maya’s schedule described a whole block of buyers at once. Where did it come from? From adding up individuals. Suppose only three people ever buy on her corner: Ana, Ben and Cara. At $1, Ana wants 3 cups an hour, Ben wants 2, and Cara wants none, because she thinks $1 is too much. The market demand at $1 is 3 + 2 + 0 = 5 cups. Notice you add across the row, at one fixed price. You never add prices together.

Now drop the price to 50 cents. Ana wants 4 cups, Ben wants 3, and Cara finally wants 2. Market demand at 50 cents is 4 + 3 + 2 = 9 cups. Raise the price to $2 and Ana wants 1, Ben wants 1, Cara wants 0, for a market total of 2. Do this at every price and you have the market demand schedule. Each row is a sum of individual demand at that price.

This adding-up explains something important. Market demand slopes down for two reasons at once. Each person buys less as the price rises, and some people drop out entirely. Cara bought nothing at $1 and 2 cups at 50 cents. Lower prices bring in new buyers who were on the sidelines. The more buyers a market has, the larger the quantity demanded at every price, so more buyers means a bigger schedule from top to bottom.

Real markets have thousands or millions of buyers, so no one builds the table by hand. Companies estimate it from sales data, surveys and experiments. A streaming service might test a $2 price increase in one region and watch how many subscribers cancel. What it learns is a piece of the market demand schedule. The logic is exactly the one you just did with Ana, Ben and Cara.

Deshawn kept a notebook too, but his answered a different question: at each price, how many cups am I willing to make and sell? At 50 cents he would bother with about 10 cups an hour. It barely covered his lemons. At $1 he would make 30. At $1.50 he would make 45, at $2 he would make 60, and at $2.50 he would squeeze lemons as fast as he could for 70. This list is a supply schedule.

His notebook shows the law of supply. When the price of a good rises and nothing else changes, sellers offer more of it. When the price falls, they offer less. Price and quantity supplied move in the same direction. Quantity supplied is the specific amount sellers offer at one specific price. At $1 Deshawn’s quantity supplied was 30 cups. At $2 it was 60.

The same word warning applies here. Supply is the whole schedule, the full relationship between price and the amount offered. Quantity supplied is one row. If a news report says "the supply of eggs is 200 million dozen," an economist would ask "at what price?" Supply is the list. Quantity supplied is the number at one price on that list.

Demand slopes down and supply slopes up, and that difference is the heart of every market. Buyers want more when it is cheap. Sellers want to sell more when it is expensive. The two sides pull against each other, and the price is where they settle. Before we get to that meeting point in the next chapter, we need to know why sellers behave this way.

Why did Deshawn offer 30 cups at $1 but 60 at $2? Because the extra cups cost him more to make. The first 30 cups used the lemons already in his fridge and the hour he had free anyway. To make 60, he had to bike to the store, buy lemons at a higher price, borrow a second cooler, and stand in the heat through the afternoon he had planned to spend at the pool. Each additional cup had a higher marginal cost than the one before it.

A seller keeps making units as long as the price covers the marginal cost. Suppose the 31st cup costs 90 cents to make once you count the trip to the store and the lost pool time. At a price of $1 it is barely worth it. The 50th cup might cost $1.40 in lemons and effort. Nobody makes that cup at $1, because it would lose money. At $2 it earns 60 cents of profit, so it gets made. Higher prices unlock the costly units.

There is a second reason supply rises with price. Higher prices bring in new sellers. On the first Saturday there were two stands on Maple Street. If lemonade sold reliably for $2.50, three more kids would set up tables by the next weekend. Market supply, like market demand, is a sum across everyone, and a high price makes the sum larger because more producers join in.

Adam Smith made this point in 1776. He said that we do not get our dinner from the kindness of the butcher or the baker. We get it because selling dinner serves their own interest. That is not a cynical claim. It is an observation that when buyers will pay more, sellers find it worth their while to produce more. The upward slope of supply is self-interest showing up in a table.

Here is the single most common mistake in a first economics class. On the hot Saturday, people wanted more lemonade. Students often say "demand went up, so the price went up, so demand went down again." That sentence uses the word demand for two different things and ends up chasing its own tail. Keep two ideas apart. A change in the good’s own price causes a change in quantity demanded, a move from one row of the schedule to another. A change in anything else causes a change in demand, a new schedule.

Look at Maya’s numbers. On a mild day she sold 40 cups at $1 and 15 at $2. Moving from $1 to $2 is a movement along the curve. The list did not change; she just read a different row. Now the heat wave hits. At $1 she could have sold 70 cups. At $2 she could have sold 35. Every row got bigger. That is a shift of the whole demand curve to the right. Nothing about her price caused it. The weather did.

A quick test: ask what changed first. If the good’s own price changed and buyers responded, that is a movement along the curve. If the weather, incomes, tastes, or the price of some other good changed, that is a shift. Price changes never shift a good’s own demand curve. They only move you along it. Price does its work at a different step, when the two sides of the market meet.

The same rule applies to supply. When lemons got expensive, Deshawn offered fewer cups at every price. That is a leftward shift of supply, a change in supply. When the price of lemonade itself rose from $1 to $1.50 and he made 45 cups instead of 30, that was a movement along his supply curve, a change in quantity supplied. The rest of this chapter lists the things that shift each curve.

Tariq gets his first job stocking shelves and starts bringing home about $200 a month. His shopping changes right away. He buys more fresh sandwiches from the deli, more concert tickets, and a better pair of shoes. For these goods a rise in income shifted his demand curve to the right. He wants more at every price. Economists call these normal goods. Most goods are normal goods.

Something else happens. Tariq buys fewer packs of instant noodles and rides the bus less, because he can split rides with friends now. For these goods his demand curve shifted left when his income rose. They are inferior goods. Inferior does not mean bad. It means people buy less of them as they get richer and more of them when money is tight. Store-brand cereal, used textbooks, and secondhand clothes are common examples.

Whether a good is normal or inferior depends on the buyer and the situation, not on the good itself. For a family that never eats out, a fast-food burger is a treat and a normal good. For a family that eats at restaurants often, a fast-food burger might be what they buy when money is short, an inferior good. The test is always the same question: when income rises, does demand for this thing shift right or left?

This matters beyond one shopper. When a recession cuts many people’s incomes at once, demand for normal goods like new cars and vacations shifts left across the whole economy. Demand for inferior goods like discount groceries shifts right. Businesses plan around this. A discount grocery chain often does well in hard times, and an airline often does badly, for the same underlying reason.

A block away from Maya’s stand, a gas station sells iced tea. Iced tea and lemonade are substitutes: two goods that do a similar job, so buyers pick one or the other. On a day when the station’s iced tea costs $1, Maya sells 40 cups of lemonade at $1. If the station raises iced tea to $2, some tea drinkers walk to Maya’s table instead. She now sells 60 cups at her same $1 price. Her demand curve shifted right, and she did not change a thing.

The rule: when the price of a substitute rises, demand for the other good shifts right. When the price of a substitute falls, demand shifts left. Notice that nothing here is about the price of lemonade itself. It is the price of a different good that moved Maya’s curve. Beef and chicken, streaming services and movie tickets, and two brands of phone are all substitute pairs.

Complements work the opposite way. These are goods used together, like hot dogs and buns, printers and ink, or lemonade and the cookies Deshawn sells next to his pitcher. If Deshawn raises his cookie price from 50 cents to $1, fewer people stop by for the cookie-and-lemonade combo. His lemonade sales fall at every lemonade price. A rise in the price of a complement shifts demand for the other good to the left.

Students sometimes confuse this with the substitution effect from the law of demand. The substitution effect is about the good’s own price rising and buyers switching away. That is a movement along the curve. A change in a related good’s price, holding the good’s own price fixed, is a shift. Ask yourself whose price changed. If it was a different good’s price, the curve shifted.

Three more things move the demand curve. The first is tastes, or preferences. A video of a Chicago chef making "the perfect lemonade" goes viral in June, and suddenly everyone on Maple Street wants some. Maya sells 55 cups at $1 instead of 40. Nothing about price or income changed. People just wanted lemonade more. Tastes shift for many reasons: fashion, health news, advertising, a hot new artist, or simply a hot day.

The second is expectations about the future. If buyers think a price will be higher next week, they buy more now. Before a hurricane, people fill their gas tanks and buy bottled water today, shifting demand right today. If buyers expect a price to drop, like a phone whose new model is about to be announced, they wait, and demand today shifts left. Expected changes in income work the same way. A student who expects a summer job may spend a bit more in May.

The third is the number of buyers. Market demand is the sum of individual demands, so more buyers means a bigger sum at every price. A block party brings 200 people to Maple Street and Maya’s curve shifts far to the right. A neighborhood that loses population sees demand for housing, haircuts and groceries shift left. Population growth, tourism, and a school opening nearby all work through this channel.

Here is the full list of demand shifters: income, prices of related goods, tastes, expectations, and the number of buyers. When you read about a market, run through the list and ask which one changed. If the answer is "the good’s own price," it is not on the list, because that is a movement along the curve. Every item on this list moves the whole curve.

Everything a seller uses to make a good is an input: lemons, sugar, cups, ice, and Deshawn’s time. On a normal week each cup costs him about 30 cents in supplies. Then the store doubles the price of lemons from 25 cents to 50 cents each. Now each cup costs about 40 cents. His cost of production rose, and at any given lemonade price he keeps less per cup. At $1 he used to make 30 cups. Now he only bothers with about 18. Every row of his supply schedule shrank, a shift to the left.

The logic follows from the last section. A seller makes a unit when the price covers its marginal cost. If costs rise, fewer units clear that bar at any given price. Higher wages, pricier raw materials, higher rent, or higher energy bills all push supply left. When costs fall, supply shifts right. A drop in the price of lemons would let Deshawn offer more cups at every price.

Technology is the other big supply shifter. Deshawn’s aunt lends him an electric juicer. He can squeeze a lemon in five seconds instead of a minute, so the labor cost of each cup drops. At $1 he is now willing to make 50 cups instead of 30. Better technology lowers the cost of each unit, and the supply curve shifts right. This is why prices of things like televisions and calculators have fallen for decades while their quality has risen.

In agriculture, better seeds, GPS-guided tractors and irrigation have let Illinois farmers grow far more corn and soybeans per acre than their grandparents did. That is a rightward shift in the supply of corn: at any given price, more bushels come to market. Note the direction. Technology almost never shifts supply left, because a seller is free to ignore a tool that does not help.

Suppose the village passes a rule: every cup of lemonade sold on a public sidewalk owes a 20-cent tax. To Deshawn this feels exactly like a cost increase. A cup that cost 40 cents to make now costs 60 cents once the tax is paid. At every price he offers fewer cups, so supply shifts left. A tax on sellers works like a higher input cost. Governments tax gasoline, cigarettes and alcohol partly to raise money and partly because they know it shrinks supply.

A subsidy is the reverse: a payment from the government to producers for each unit they make. If the village paid 20 cents per cup to encourage kids’ businesses, Deshawn’s cost per cup would effectively fall to 20 cents, and he would offer more at every price. Supply shifts right. The federal government subsidizes some crops and some forms of energy for this reason. Whether a given subsidy is a good idea is a separate question that economists debate.

Sellers have expectations too. If Deshawn hears that a heat wave will push lemonade to $2.50 next Saturday, he might make less this Saturday and save his lemons, shifting today’s supply left. A farmer who expects higher wheat prices in the spring might store grain instead of selling it in the fall. When sellers expect lower prices ahead, they sell more now, shifting today’s supply right.

Finally, the number of sellers. Market supply is a sum, so a third stand on Maple Street shifts market supply right. A dozen new coffee shops shift the supply of coffee in a neighborhood right. When a factory closes or a chain shuts stores, supply shifts left. The full list of supply shifters: input costs, technology, taxes and subsidies, expectations, and the number of sellers. As with demand, the good’s own price is not on the list.

Go back to the summer on Maple Street with the tools you now have. On the second Saturday the temperature hit 96 degrees. Which curve moved? Buyers wanted more lemonade at every price, so demand shifted right. Which shifter? Tastes, because the weather changed how much people wanted a cold drink. Nothing about the sellers’ costs changed. Maya ran out at $1 because the old price no longer matched the new demand.

On the third Saturday, lemons doubled in price. Which curve moved? Sellers offered fewer cups at every price, so supply shifted left. Which shifter? Input costs. Buyers had not changed at all. Both Saturdays ended with a higher price for lemonade, but for opposite reasons. In the next chapter you will see that the two cases have different effects on the number of cups sold, which is how economists tell them apart in real data.

A useful habit is to ask three questions in order. First, which curve: is this about buyers or about sellers? Second, which direction: does the change make people want more (right) or less (left) at every price? Third, which shifter from the list: income, related goods, tastes, expectations or number of buyers for demand; input costs, technology, taxes and subsidies, expectations or number of sellers for supply. If your answer to "which shifter" is "the price of the good itself," stop. That is a movement along a curve, not a shift.

Economists call this whole approach holding other things equal, or ceteris paribus, a Latin phrase that means just that. You change one thing at a time and trace its effect. Real life changes many things at once, which is why the news can be confusing. The skill you are building is to pull one thread at a time. Each of the determinants of demand and supply is one thread.

Put a demand schedule and a supply schedule side by side and something jumps out. Consider tomatoes at a Saturday farmers market, priced per pound. At $1, buyers want 500 pounds but sellers bring only 100. At $2, buyers want 400 and sellers bring 200. At $3, buyers want 300 and sellers bring 300. At $4, buyers want 200 and sellers bring 400. At $5, buyers want 100 and sellers bring 500. Only one row matches: $3, where 300 pounds are wanted and 300 are offered.

That price is the equilibrium price, and 300 pounds is the equilibrium quantity. Equilibrium is a word borrowed from physics. It means a resting point, a place where the forces balance. At $3 every buyer who is willing to pay finds a seller, and every seller who is willing to sell finds a buyer. No one is left holding tomatoes and no one goes home empty-handed who was willing to pay the going price.

A market has no manager who sets this price. It emerges. If sellers try $5, they will have 400 unsold pounds at the end of the day and will cut prices. If they try $1, they will sell out by 9 a.m. and notice they could have charged more. Both mistakes push the price back toward $3. Economists call this the market-clearing price, because at $3 the market clears: what is brought is what is bought.

The economist Alfred Marshall, writing in 1890, compared demand and supply to the two blades of a pair of scissors. Asking whether the buyers or the sellers set the price is like asking which blade does the cutting. Both do, together. Any change on either side moves the point where the blades cross. The rest of this chapter follows those changes.

Suppose the tomato sellers, for whatever reason, price at $2. Buyers want 400 pounds. Sellers brought 200. The gap, 400 minus 200, is a shortage of 200 pounds. A shortage is the amount by which quantity demanded exceeds quantity supplied at a particular price. It is not the same as scarcity, which is always true of everything. A shortage exists only at a price below equilibrium, and it disappears when the price rises.

What happens next in a real market? Some buyers who really want tomatoes offer more. A seller with a short line and an eager crowd raises the sign to $2.50. As the price climbs, two things happen at once. Some buyers drop out, so quantity demanded falls from 400 toward 300. Sellers bring more from the truck, so quantity supplied rises from 200 toward 300. At $3 the gap is gone. The shortage itself was the pressure that fixed it.

While the shortage lasts, the market needs another way to decide who gets the tomatoes. Usually it is first come, first served: a line. It can also be luck, connections with the seller, or a limit of two per customer. Economists call this non-price rationing. It is what Jada ran into on the ticket site. At $75 there were far more buyers than seats, so speed and luck did the choosing, not price.

Back at the arena, the shortage was enormous. Sixty thousand fans wanted a seat at $75 and 18,000 seats existed. The shortage was 42,000. The resale site was where the price finally rose toward equilibrium and buyers who valued the seat most, or had the most money, got them. Whether that outcome is fair is a real argument. That it happened is simply what shortages do.

Now flip it. Sellers price tomatoes at $4. Buyers want only 200 pounds, but sellers brought 400. The extra 200 pounds is a surplus: the amount by which quantity supplied exceeds quantity demanded at a particular price. Surplus is the mirror image of shortage. It appears only when the price is above equilibrium, and it goes away when the price falls.

Sellers do not enjoy watching tomatoes rot. One seller marks down to $3.50 to move product. Others follow. As the price falls, quantity demanded rises from 200 toward 300, and some sellers decide not to bring as much next week, so quantity supplied falls from 400 toward 300. The price settles at $3 and the surplus is gone. Clearance racks, end-of-season sales and "buy one get one" deals are all surplus at work.

A surplus is also what unsold inventory looks like on a company’s books. When a video game console sits on shelves for months, the retailer has priced above what buyers will pay at that quantity. When it cuts the price, it is not being generous. It is moving toward equilibrium. When you see a big markdown, ask which side of the market misjudged, and by how much.

Here is the whole picture in one line. Below equilibrium, shortage, price rises. Above equilibrium, surplus, price falls. At equilibrium, neither, price stays. In a market where the price is free to move, it keeps getting pushed back toward the crossing point. The next chapter looks at what happens when a law stops the price from moving. This chapter stays with prices that can.

Go back to lemonade on Maple Street and give it a supply schedule too. Before the heat wave, buyers wanted 60 cups at 50 cents, 40 at $1, 25 at $1.50 and 15 at $2. Sellers offered 10 at 50 cents, 30 at $1, 45 at $1.50 and 60 at $2. There is no exact match, but the crossing is between $1 and $1.50: at $1 there is a shortage of 10 cups, and at $1.50 a surplus of 20. Say the equilibrium is about $1.15 and about 35 cups.

Now the heat wave shifts demand right. Buyers want 70 cups at $1, 50 at $1.50 and 35 at $2. Supply has not changed. At the old price of about $1.15 there is now a big shortage, so the price rises. At $1.50, buyers want 50 and sellers offer 45, close to a match. The new equilibrium is roughly $1.50 and about 47 cups. Compare the two: price went up and quantity went up. That is the first case, a rightward shift of demand.

The second case is the reverse. A cold snap in August shifts demand left. Fewer cups are wanted at every price. At the old price there is a surplus, so the price falls, and at the lower price sellers offer fewer cups. Price down, quantity down. A leftward shift of demand lowers both. Whenever price and quantity move in the same direction, suspect that demand shifted.

This is how economists read the news. If a report says a good’s price rose and more of it sold, the demand curve moved right. Higher price plus more sales cannot come from a supply shift, because a supply shift pushes price and quantity in opposite directions. The direction of price alone is not enough. You need both numbers to tell which curve moved.

Now hold demand fixed and move supply. On the third Saturday the price of lemons doubled. Deshawn and Maya offered fewer cups at every price: 18 at $1, 30 at $1.50, 40 at $2 instead of 30, 45 and 60. Demand is the mild-day schedule: 40 at $1, 25 at $1.50, 15 at $2. Before, the equilibrium was about $1.15 and 35 cups. Now at $1.15 there is a shortage, so the price rises. At $1.50 buyers want 25 and sellers offer 30, close. Call the new equilibrium about $1.40 and about 28 cups.

Notice the pattern. Price went up, but quantity went down. That is a leftward shift of supply: fewer cups at every price, so the market clears at a higher price and a smaller quantity. Compare it with the heat wave, where price went up and quantity went up. Same direction for price, opposite direction for quantity. That difference is how you tell a demand shift from a supply shift when both raise the price.

The other case is a rightward shift of supply. Deshawn’s electric juicer lowers his cost, or Leo opens a third stand, and more cups are offered at every price. At the old price there is a surplus, so the price falls, and at the lower price buyers move down their demand curve and buy more. Price down, quantity up. This is what decades of better technology did to the price of a calculator or a flat-screen TV.

Put the four cases in a small table in your head. Demand right: price up, quantity up. Demand left: price down, quantity down. Supply right: price down, quantity up. Supply left: price up, quantity down. A quick way to remember: demand shifts move price and quantity the same way; supply shifts move them opposite ways. Practice on real events until it feels automatic.

Real events often push both curves at the same time. Take the worst week of the summer on Maple Street: the heat wave hits and the price of lemons doubles in the same week. Demand shifts right, which pushes price up and quantity up. Supply shifts left, which pushes price up and quantity down. Both shifts push the price up, so the price definitely rises. But one shift pushes quantity up and the other pushes it down. Quantity could go either way.

Which way quantity goes depends on which shift is bigger. If the heat wave is huge and the lemon price rise is small, the demand shift wins and quantity rises. If the heat is mild and lemons are suddenly unaffordable, the supply shift wins and quantity falls. Without knowing the sizes, an economist says quantity is indeterminate: it cannot be determined from the directions alone. That is an honest answer, not a dodge.

There are four two-curve combinations. Demand right and supply left: price up for sure, quantity uncertain. Demand left and supply right: price down for sure, quantity uncertain. Demand right and supply right: quantity up for sure, price uncertain. Demand left and supply left: quantity down for sure, price uncertain. In every case, one variable is certain because both shifts agree on it, and the other is uncertain because they disagree.

The spring of 2020 was a giant version of this. Demand for many things like restaurant meals and airline seats shifted sharply left. At the same time, supply of many goods shifted left as factories and ports slowed. For some products prices rose, for others they fell, depending on which curve moved more. When the news seems contradictory, sort each event into its curve and direction. The contradictions usually turn out to be two shifts at once.

The law of demand says buyers buy less when the price rises. It does not say how much less. That is what elasticity measures. Maya raises her lemonade from $1.00 to $1.20. That is a 20% price increase, because 0.20 divided by 1.00 is 0.20. Her sales fall from 40 cups to 30 cups an hour, a 25% drop, because 10 divided by 40 is 0.25. The price elasticity of demand is the percent change in quantity divided by the percent change in price: 25% divided by 20% equals 1.25.

Read that number as "quantity moved 1.25 times as much as price did, in percent." Economists drop the minus sign, since quantity and price always move opposite ways for demand. When the number is greater than 1, demand is elastic: buyers respond strongly, like a stretchy rubber band. When it is less than 1, demand is inelastic: buyers barely budge. When it is exactly 1, demand is unit elastic.

Now try gas. A station raises its price from $3.00 to $3.60 a gallon, a 20% increase. Deshawn’s dad still has to drive to work, so he cuts back from 10 gallons a week to 9.5, a 5% drop. Elasticity is 5% divided by 20%, which is 0.25. Inelastic. The same 20% price rise that cost Maya a quarter of her customers cost the gas station almost none of its gallons.

Two warnings. First, elasticity uses percents, not raw amounts, so you can compare lemonade cups with gallons of gas. Second, this is the simple method used in introductory classes. Economists often use a midpoint formula that gives the same answer whether the price goes up or down. Your teacher will tell you which to use. Either way, the meaning is the same: a big number means buyers are sensitive to price, and a small number means they are not.

Why did lemonade buyers flee a 20% price rise while gas buyers barely noticed? Four things decide how elastic demand is. The first and biggest is substitutes. Lemonade has many: iced tea, water, soda, juice. When the price rises, buyers have somewhere to go. Gas, for someone who has to drive to work, has almost none in the short run. The more and closer the substitutes, the more elastic the demand. That is also why the demand for one brand of gas is elastic even though the demand for gas in general is not.

The second is share of budget. A 20% rise in the price of table salt costs a family a few cents a year, so nobody changes anything. A 20% rise in rent might be $200 a month, so families move, get roommates, or look farther from the city. Goods that take a big slice of income tend to have more elastic demand, because a price change there really hurts and buyers respond.

The third is necessity versus luxury. Insulin for someone with diabetes is a necessity and its demand is very inelastic. A weekend at a water park is a luxury; if the price doubles, most families skip it. Note that necessity is about the buyer’s situation, not the product’s dignity. Gas is a necessity for a commuter with no bus line and a luxury for someone who mostly bikes.

The fourth is time. Right after gas prices jump, drivers can do little, so demand is inelastic. Over a year, they can carpool, buy a more efficient car, move closer to work, or take the train. Demand becomes more elastic the longer buyers have to adjust. When you estimate elasticity, always ask: elastic over what time period? The answer for the same good can be 0.2 in a month and close to 1 over several years.

A seller thinking about a price change wants to know one thing: will I take in more money or less? Total revenue is price times quantity sold. Maya at $1.00 sold 40 cups, so her total revenue was $40. At $1.20 she sold 30 cups, for $36. She raised her price and took in less money. That happened because her demand was elastic (1.25). Buyers left faster, in percent, than the price rose, so the lost sales outweighed the higher price per cup.

The gas station had the opposite experience. At $3.00 Deshawn’s dad bought 10 gallons a week: $30. At $3.60 he bought 9.5 gallons: $34.20. Revenue rose. With inelastic demand (0.25), the price rise gained more than the tiny drop in gallons lost. The rule: when demand is elastic, price and total revenue move in opposite directions. When demand is inelastic, price and total revenue move in the same direction. When demand is unit elastic, revenue barely changes.

Look at a small concert venue that seats 100 and is testing prices. At $5 it sells 100 tickets for $500. At $10, 80 tickets for $800. At $15, 60 tickets for $900. At $20, 40 tickets for $800. At $25, 20 tickets for $500. Revenue rises as the price climbs from $5 to $15, so demand is inelastic in that range. Revenue falls from $15 to $25, so demand is elastic there. Along one straight demand curve, elasticity changes: inelastic at low prices, elastic at high prices.

This is the revenue test, and businesses use it constantly. A streaming service raising its monthly fee is betting demand is inelastic. A restaurant running a half-price Tuesday is betting demand that night is elastic. Neither needs the formula. If revenue goes up after a price increase, demand was inelastic. If it goes down, demand was elastic. The formula and the revenue test always agree.

Sellers have an elasticity too. Price elasticity of supply is the percent change in quantity supplied divided by the percent change in price. When lemonade rose from $1.00 to $1.50, a 50% increase, Deshawn’s cups rose from 30 to 45, also a 50% increase. His elasticity of supply was 50% divided by 50%, which is 1.0. The same labels apply: above 1 is elastic, below 1 is inelastic.

What decides it? Mostly how easily sellers can make more, and that depends on time. A Van Gogh painting has an elasticity of supply of zero: raise the price all you want and there will never be another. Economists call that perfectly inelastic. Arena seats for one night are almost the same: 18,000 exist, and the price cannot change that number this week. Over a year, though, the promoter could add a second night, so supply becomes more elastic when the time period is longer.

Illinois corn shows the same pattern. In the middle of the growing season, a price jump changes almost nothing; the seeds are already in the ground. By next spring, farmers can plant more acres of corn and fewer of soybeans, so the yearly quantity responds a good deal. Across several years, new land can be rented and new equipment bought. The longer the time, the more elastic the supply.

Elasticity of supply also depends on whether inputs are easy to get. Lemonade is elastic because lemons, sugar and cups are easy to buy more of. A restaurant that needs a licensed chef, a kitchen and a permit cannot double its meals overnight. Goods that need scarce or slow-to-build inputs have inelastic supply. When you see a price spike that lasts, it usually means supply could not respond quickly.

Elasticity is not just a number for tests. Imagine you run a small bakery in Peoria and your flour costs jump. Should you raise your bread price by 10% or eat the cost? If your bread is much like the bread at three other bakeries nearby, demand is elastic, and a 10% rise might cost you 20% of your customers and cut revenue. If you are the only bakery for miles and people love your bread, demand is inelastic, and the price rise mostly sticks. The right choice depends on the elasticity.

Governments use the same reasoning. A tax on cigarettes raises a lot of money precisely because demand for cigarettes is inelastic: people keep buying even when the price rises, so the tax collects steadily. The same inelasticity means the tax does less to cut smoking than its supporters hope, at least in the short run. A tax on a good with elastic demand does the opposite: it cuts sales a lot and collects little. Economists point this out whichever goal a lawmaker has.

Elasticity also explains why some price spikes hurt more than others. When gas rises 20%, families cannot cut back much, so their weekly bill really goes up. When strawberries rise 20%, families buy blueberries or skip fruit that week, and the bill barely moves. Inelastic goods are the ones where a price rise lands on the consumer and stays there. Elastic goods are the ones where a price rise is mostly the seller’s problem.

Finally, elasticity is a prediction tool for the two-curve stories in this chapter. When supply shifts left, how much the price rises depends on how elastic demand is. If demand is inelastic, like gas, a small supply cut causes a big price jump because buyers will not back off. If demand is elastic, like one brand of soda, the same supply cut barely moves the price because buyers walk away. This is why gasoline prices swing so wildly while soda prices do not.

Now put the whole chapter on Jada’s ticket. Supply: 18,000 seats, perfectly inelastic for that night. Demand: about 60,000 fans at $75, about 18,000 at $250. The equilibrium price is around $250, because that is where the quantity fans want equals the seats that exist. The artist charged $75, far below equilibrium. At $75 the shortage was about 42,000. Something other than price had to decide who got in, and on Friday morning that was the online queue.

Why would an artist leave so much money on the table? There are real reasons. Some want younger fans and less wealthy fans to have a chance, and a low face value plus a random queue does that better than a $250 price. Some fear the bad press of being called greedy. Some want a guaranteed sellout and a full, loud room. These are choices about who gets the seats, and they are legitimate. But the shortage they create does not go away. It moves.

It moves to the resale market, where the price is free. Sellers who got tickets at $75 offer them at $220, $340 or $900, and buyers who value the show most, or simply have the most money, take them. Elasticity explains the size of the jump. Supply is fixed, so it cannot respond at all. Demand for one specific singer on one specific night has few substitutes and is fairly inelastic, so it takes a very high price to trim 60,000 eager fans down to 18,000 seats. A four-times markup is what that looks like.

Compare milk. If a grocery store sold milk at half the going price, it would run out, but nobody would resell milk at four times the price, because the store down the street has plenty and supply is elastic. The ticket story is dramatic because both curves are extreme: perfectly inelastic supply and inelastic demand. Whether resale should be limited, or whether artists should just charge more, is a debate with good arguments on both sides. The economics tells you what will happen either way. It does not tell you what is fair.