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. 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.