So slope is equal to the rate of change of our vertical variable over the rate of change of our horizontal variable. It's sometimes described as rise over run. And for any line, it's associated with a slope because it has a constant rate of change. If you took any two points on this line, no matter how far apart or no matter how close together, anywhere they sit on the line, if you were to do this calculation, you would get the same slope. That's what makes it a line. But what's fascinating about calculus is we're going to build the tools so that we can think about the rate of change not just of a line, which we've called slope in the past, we can think about the rate of change, the instantaneous rate of change of a curve, of something whose rate of change is possibly constantly changing. So for example, here's a curve where the rate of change of y with respect to x is constantly changing.