If it was the derivative of x squared with respect to x, we'd have 2x. If it was the derivative of r of t squared with respect to r of t, it's 2r of t. But this doesn't get us just the derivative with respect to time. This is just the derivative with respect to r of t. With the derivative, which this changes with respect to time, we have to multiply this times the rate at which r of t changes with respect to time. So the rate at which r of t changes with respect to time, well, we could just write that as dr dt. These are equivalent expressions. And of course, we have our pi out front. And I just want to emphasize, this is just the chain rule right over here.