And our goal is to find the m and the b, which would define an actual line, to find the m and b that minimize the squared error. And the way that we do that is we find a point where the partial derivative of the squared error with respect to m is 0, and the partial derivative with respect to b is also equal to 0. So it's flat with respect to m. So that means that the slope in this direction is going to be flat. Let me do it in the same color. So the slope in this direction, that's the partial derivative with respect to m, is going to be flat. It's not going to change in that direction. And the partial derivative with respect to b is going to be flat.