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
http://www.grand-illusions.com/images/articles/opticalillusions/dragon_illusion/dragon.pdf
That link comes from the page in his original posting.
But it's not as impressive once I assembled it myself. I'm too aware of the 'real' structure and it's hard to look past that.
I'll glance at it later in the day (after I forget about it for awhile) and see if it fools me.
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!