And so now we can take the derivative with respect to time of both sides of this business. So the derivative with respect to time of our volume and the derivative with respect to time of this business. Well, the derivative with respect to time of our volume, we can just rewrite that as dv dt, this thing right over here. This is dv dt. And this is going to be equal to, well, we could take the constants out of this. This is going to be equal to pi over 12 times the derivative with respect to t of h to the third power. And just so that the next few things I do will appear a little bit clearer, we're assuming that height is a function of time.