Well, one thing that we know, we know that the slope of the tangent line, at least the way I've drawn it right over here, is equal to zero. So we can say f prime of c is equal to zero. And the other thing we can see is that we are concave downward in the neighborhood around x equals c. So notice our slope is constantly decreasing. And since our slope is, notice it's positive, it's less positive, even less positive, it goes to zero, then it becomes negative, more negative, and even more negative. So we know that f prime prime, we know that f prime prime of c is less than zero. And so, I haven't done any deep mathematical proof here, but if I have a critical point where f prime, where our critical point at x equals c, so f prime of c is equal to zero, and we also see that the second derivative there is less than zero, well, intuitively, this makes sense that we are at a maximum value. And we could go the other way.