A simple object that can appear as a circle, a square, or a triangle, depending on the direction from which it is viewed.
I made this after listening to some friends discussing the 3-in-1 nature of the Christian Trinity - One deity that manifests as the Father, Son, and Holy Spirit.
Regardless of whether the theological application is appropriate or not, the thing can serve as an object lesson in how something can be interpreted in different ways, depending on your point of view.

The objects horizontal cross section is an ellipse. At the base, the major and minor axis = 1. As Z increases from 0 to 1, the major axis holds constant at 1 while the minor axis decreases linearly to zero.