[MUSIC] I mentioned that elasticity of demand is always positive because we've taken the absolute value and it ranges between zero and infinity. I'd like to spend a moment talking about these two extremes, the extreme of zero and the extreme of infinity. What does an elasticity of zero mean? That means that it doesn't matter what happens to the price, the quantity demanded doesn't change. So no matter what is the percentage change in price, the percentage change in quantity is going to be zero. If you graph this, you will see that it is a completely vertical demand curve. We call this the perfectly inelastic demand. So this is the situation where the elasticity of demand is equal to zero. And what this means is, is that people will purchase the same amount of the good, no matter what the price is. It's hard to think of real world examples where the demand is perfectly inelastic. But let me give you some goods that might help you intuitively think through this. My favorite example, is always to think about things like cigarettes. If you think of cigarettes. People who are addicted to cigarettes, it doesn't matter what the price is. They will continue and purchase the same quantity. If you're the type of person that smokes maybe two packs a day, you'll purchase the two packs if the price is $1 a pack. You'll purchase two packs if the price is $2 a pack. And you'll even purchase when the price is $4 a pack. Presumably if the price is high enough, maybe you'll give up, maybe you'll try and stop smoking, maybe you'll try and get a nicotine patch instead. So you might try to change your behavior, but if you're really addicted to the product, that means the demand is perfectly inelastic. And you'll go to very long lengths to find the money to pay for that product, even when the price is very high. So again, thinking about the type of good someone might be addicted to, is a good helpful intuitive example to think about, when you're trying to think about a perfectly inelastic demand. So that's the one extreme. The other extreme is when elasticity of demand is infinite. And graphically this is a completely horizontal demand curve. And we call this a perfectly elastic demand curve. That's a situation where even a small change in price, causes an infinite change in quantity. In other words, it's an example where if the price goes up even by one cent, people will say no thank you, I'm not interested in this product. I'm going to get something else. And it must be the case that it is a good for which very close substitutes, perfect substitutes are available. Here again, it's very hard to think of real world examples, I sometimes think of two soda machines. Two machines where you put a dollar in and you get a soda can. What happens if one of these machines increases their price to $2? These two machines are sitting right next to each other. One of them costs $1, the other one costs $2. No one's going to buy their soda from the machine that asks for $2. Everyone is going to go to the one that costs $1. So that's an example where the demand for the soda cans in this environment is perfectly elastic. Because there's no ability for one machine to raise the price, because no one is going to go to that machine. Everyone is going to the other machine. So again, two extremes. Perfectly inelastic demand, which is the vertical line and the perfectly elastic demand, where elasticity of demand is equal to infinite, which is the horizontal demand curve.