There is a dragon paper model in the internet that is an example of a hollow-face illusion. You print it, fold it and you get an amazing optical illusion. When you walk around, the dragon head appears to be following you!
Then I had the idea of making the same dragon in LEGO. It’s not very pretty, since the back of a plate doesn’t look much like the front of a plate. But the illusion works! Check the video (sorry for the quality, I need a better camera :P):
The illusion happens because our brains are used to convex faces, and when it sees a hollow one it interprets it as convex. This misinterpretation makes the face appear to move.