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. The derivative of something squared with respect to time is going to be the derivative of the something squared with respect to the something, so that's 2 times the something, times the derivative of that something with respect to time. I can't emphasize enough what we did right over here. This is the chain rule.