Necker Cube Optical Illusion

First published in 1832 by Swiss crystallographer Louis Albert Necker, The Necker Cube is an ambiguous line drawing.

Is the blue face the front or back face of this cube?

Is the blue dot inside or outside the cube?

It is a wire-frame drawing of a cube in isometric perspective. This makes the picture ambiguous; it can be interpreted two different ways. When a person stares at the picture, it will often seem to flip back and forth between the two valid interpretations.