You might have been tempted to take the derivative over here with respect to h. But remember, we're thinking about how things are changing with respect to time. So we're assuming we did express volume as a function of height, but we're saying that height itself is a function of time. So we're taking the derivative of everything with respect to time. So that's why the chain rule came into play when we were taking the derivative of h or the derivative of h of t, because we're assuming that h is a function of time. Now, what does this thing right over here get us? Well, we're telling us at the exact moment that we set up this problem, we know what dv dt is. We know that it's 1 centimeter cubed per second.