Necker cube

The Necker cube: a wire frame cube with no depth cues

One possible interpretation of the Necker cube.

Another possible interpretation

The Necker cube is an optical illusion that was first published as a rhomboid in 1832 by Swiss crystallographer Louis Albert Necker.^[1] It is a simple wire-frame, two dimensional drawing of a cube with no visual cues as to its orientation, so it can be interpreted to have either the lower-left or the upper-right square as its front side.