In the vein if "tricking the eye" posts, I was pointed to this great little example of opponent-process afterimages. Just follow the directions on the screen. One thing to keep in mind that isn't mentioned though is to keep your eye on the center dot, even after the picture changes.Oh, and don't worry. It's not one of those crappy "scream" things that pop up. Heck, turn the speakers off if you want. They aren't needed for this illusion.