Find the volume of the described solid.

The base of the solid is a circular disk with radius r. Parallel cross-sections perpendicular to the base are squares.

This is a difficult problem from the text. If someone could walk me through how to do this problem using integration, I think it will help me grasp how to do the other problems similar to it.

I'm having trouble starting, and not sure what I'm supposed to do. I think I'm supposed to find the surface area of the squares with respect to their distance from the center of the disk, and evaluate it from the -r to r. But how to I find out what the area of the square is at any given distance from along the radius?