 So here's an example of how the secant line and the tangent line start to look like each other. So here I have two points on a curve, and I'm going to draw the secant line between those two points, and I also have the line tangent to the curve at that point. And so I have my secant line, which sort of looks vaguely like the tangent line, and in particular the slope of the secant line is a approximation to the slope of the tangent line. But what happens if I move that second point closer to the first point? And so we'll see what happens there. There's my second point sliding down, and as those two points get closer, the secant line and the tangent line start to look more and more like each other, and so now with that second point very, very, very close to that first point, the slope of the secant line is going to be very close to the slope of the tangent line.