We know that c is always sandwiched in between x and x plus delta x. And intuitively, you could tell that, look, as delta x approaches 0, as this green line right over here moves more and more to the left, as it approaches this blue line, the c has to be in between. And so the c is going to approach x. So we know intuitively that c approaches x as delta x approaches 0. Or another way of saying it is that f of c is going to approach f of x as delta x approaches 0. And so intuitively, we could say that this is going to be equal to f of x. Now you might say, OK, that's intuitively, but we're kind of working on a little bit of a proof here, Sal.