Certain shapes can be obtained by revolving a 2-dimensional plane around some central axis. The 'edge' of the plane furthest from this axis traces the surface of some 3-dimensional object. There are two accepted techniques to determining the volume of such an object: the method of disks and washers and that of cylindrical shells. Below is an example of the disk method.

How can you tell if such an object in real-life can be generated in this manner?