Necker cube

I recently learnt about the Necker cube and thought it would be interesting to mention it because it’s a very good way to explain the concept of a reality tunnel. Here’s an excerpt from the Wikipedia entry:

The Necker Cube is an ambiguous line drawing. It is a wire-frame drawing of a cube in isometric perspective, which means that parallel edges of the cube are drawn as parallel lines in the picture. When two lines cross, the picture does not show which is in front and which is behind. This makes the picture ambiguous; it can be interpreted two different ways.