And so this intuition that we hopefully just built up is what the second derivative test tells us. So it says, hey, look, if we're dealing with some function f, let's say it's a twice-differentiable function, so that means that over some interval, so that means that you could find its first and second derivatives are defined. And so let's say there's some point, x equals c, where its first derivative is equal to zero. So the slope of the tangent line is equal to zero. And the derivative exists in a neighborhood around c, and most of the functions we deal with, if it's differentiable at c, it tends to be differentiable in the neighborhood around c. And then we also assume that the second derivative exists, it's twice-differentiable. Well then, we might be dealing with a maximum point, we might be dealing with a minimum point, or we might not know what we're dealing with. And it might be neither a minimum or a maximum point.