This little dragon appears to be looking at you. But if you rotate the base this way or that the dragon's head appears to turn the other way...
Or if you stand the dragon on a shelf and walk about, the dragon's head follows you around the room. And yet it is simply a sheet of paper, cut and folded in a certain way. How does it work?
When we see a solid object rotating, there are all sorts of clues that tell us what is going on, which way it is rotating, etc. The dragon gives us the wrong clues, because we mis-interpret what its shape is. The nose of the dragon appears to be pointing out towards the viewer, but in fact the dragon's head is concave. Have a look at the dragon from the back...
http://www.grand-illusions.com/dragon.htm