So that is our volume. Now what we want to do is relate how fast the volume is changing with respect to time and how fast the height is changing with respect to time. So we care with respect to time. Since we care so much about what's happening with respect to time, let's take the derivative of both sides of this equation with respect to time. And just so I have enough space to do that, let me move this over to the right a little bit. So I just move this over to the right. And so now we can take the derivative with respect to time of both sides of this business.