So for example, here's a curve where the rate of change of y with respect to x is constantly changing. Even if we wanted to use our traditional tools, if we said, okay, we can calculate the average rate of change, let's say between this point and this point, well, what would it be? Well, the average rate of change between this point and this point would be the slope of the line that connects them. So it'd be the slope of this line, of the secant line. But if we pick two different points, if we pick this point and this point, the average rate of change between those points all of a sudden looks quite different. It looks like it has a higher slope. So even when we take the slopes between two points on the line, the secant lines, you can see that those slopes are changing.