Calculus offers two methods of computing volumes of solids of revolution obtained by revolving a plane region about an axis. These are commonly referred to as the disc/washer method and the method of cylindrical shells, which is shown in this Demonstration. In this example the first quadrant region bounded by the function and the axis is rotated about the axis. You can see the solid of revolution or an approximation by a set of cylindrical shells. As the number of shells is increased you can see that the approximation becomes closer to the solid. You can approximate the volume using shells whose heights are given by the function value at the left, right, or center of the axis interval that generates the shell.