I'm a sucker for optical illusions. I found a online store full of them, and I immediately put my wallet far from me, so I didn't buy a load of nifty-though-useless junk. I did, however, find this nifty one that's free:
He perches inside my office, watching my door. Neat, huh? What's the illusion, you ask? Watch, as he follows you:
As you go around, his head and eyes follow you around my office. However, the further you go, the more you run the risk of seeing too much:
As you go far to the right, you see the reality: the face, is in fact, inside out. It's hollowed out. Your brain, when it sees it, makes an assumption-one that you can't get around, even if you consciously know the face is inverted, you can't look at it without your brain making those assumptions. It's like a visual logical fallacy. Neat, huh?