We've featured this "hollow face" dragon paper sculpture illusion a while ago, but recently I ran across a video of it and was just as astounded as I was back then. Here it is: hit play or go to http://video.google.com/videoplay?docid=7338400765555658225&hl=en [Google Video] - via Illusions
Basically, what ruins it is that our eye figures out theres a discrepancy between the shadow were seeing and the shadow that should be there.
Try it!