Necker Cubes. This is an example of two identical necker cubes, the one on the left showing an intermediate object (blue bar) going in "down from the top" while the one on the right shows the object going in "up from the bottom" which shows how the image can change its perspective simply by changing which face (front or back) appears behind the intervening object.
Necker cube: Fixate on the red appearing dot. The two dimensional drawings give an appearance of three dimensional cubes. Ambiguity exists in the three dimensional information and as you continue to look, you will see a shift. Note which surface appears closest in each cube. Do the cubes shift independently or as a unit? What is the importance of the way that they shift?