I've just spent a happy half hour glancing through Optical Illusions and Visual Phenomena Some of the items on this page made me dizzy! I was quite pleased to see the classic face illusion. I have a transparent block with mine and monica's faces etched into it which exhibits this same effect. If asked, and I can work out how to capture the video reasonably successfully, then I may put an rotating animation of this online. (additional: I'm not the only one who has been looking at this stuff today, I clicked on this link by chance from the Movable Type 'recently updated' section).