The Necker Cube is a classic visual puzzle that has been around since the 19th century. It consists of a two-dimensional line drawing of a cube, which can be interpreted in two ways. The challenge is to switch between the two interpretations, and to try to
From Wikipedia
The Necker cube is an optical illusion that was first published as a rhomboid in 1832 by Swiss crystallographer Louis Albert Necker. It is a simple, wire-frame, two-dimensional drawing of a translucent 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.