The volume of water is going to be equal to 1 third times the area of the surface of the water, area of water surface times our height of the water, so times h. So how can we figure out the area of the water surface, preferably in terms of h? Well, we see right over here the diameter across the top of the cone is 4 centimeters, and the height of the whole cup is 4 centimeters. And so that ratio is going to be true at any depth of water. It's always going to have the same ratio between the diameter across the top and the height, because these are lines right over here. So at any given point, the ratio between this and this is going to be the same. So at any given point, the diameter across the surface of the water, if the depth is h, the diameter across the surface of the water is also going to be h. And so from that, we can figure out what the radius is going to be. The radius is going to be h over 2.