Now you see it... (wonk-eye filter)
Click to viewThis is neat:
"This kind of optical illusion, which I named "Impossible Motion", was discovered in my engineering research for machine interpretation of line drawings. In order to check the behavior of my software, I gave what are called "pictures of impossible objects" as the input expecting the software to judge them impossible. However, my software sometimes interprets "pictures of impossible objects" as actual solids, from which I found that some of "pictures of impossible objects" are not impossible; they can be constructed although general angles are used where they look rectangular."