Instead of saying it has a depth of dx, let's say it has a depth of dy. So this depth right over here is dy. So what is the volume of this disk in terms of y? And as you can imagine, we're going to do this definite integral with respect to y. So what's the volume of this thing? Well, like we did in the last video, we have to figure out the area of the top of each of these disks, or I guess you could say the face of this coin. Well, to find that area, it's pi r squared.