We're talking about the points in between. So if you have a point inside of an interval, it's going to be a minimum or maximum, and we see the intuition here. If you have non-endpoint min or max at, let's say, x is equal to a. So if you know that you have a minimum or maximum point at some point x is equal to a and x isn't the endpoint of some interval, this tells you something interesting, or at least we have the intuition. We see that the derivative at x is equal to a is going to be equal to 0, or the derivative at x is equal to a is going to be undefined. And we see that in each of these cases. Derivative is 0.