Hands in the darkness
Seeing your hands in perfect darkness is an illusion brought about by the fact that you know where they are and your brain is mapping them into your vision.

Go somewhere absolutely black, like a deep cave, and hold your hands in front of your face. You will swear you can just make them out, but in fact it's a trick of the mind. You can prove this because you can't see other people's hands, even if they put them in front of yours. Very cool.
(Tomble Underground, overground, Tombling free., Mon 7 Jul 2008, 6:48, Reply)