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. So we're left with pi times this is equal to the derivative of our area with respect to time. Now, let me rewrite all of this again, just so it cleans up a little bit. So we have the derivative of our area with respect to time is equal to pi times, actually, let me put that 2 out front, is equal to 2 times pi times, I can now switch back to just calling this r. We know that r is a function of t, so I'll just write 2 pi times r times dr dt.