So we just figured out that if f of x, this is a big result, this is exciting, that if f of x is equal to x squared, f prime of x is equal to 2x. That's what we just figured out. And I want to make sure you understand how to interpret this. f of x, if you give me a value, is going to tell you the value of the function at that point. f prime of x is going to tell you the slope at that point. Let me draw that. Because this is a key realization, and it's kind of maybe initially unintuitive to think of a function that gives us the slope at any point of another function.