The Waterfall Effect
What to see and do:Stare at the dot in the center of the spiral, and after 30 or more seconds look at you hand and/or the keyboard of you computer.
What's happening: When your brain stares at something with a constant motion it grows used to that motion, and expects everything else to have that motion. When you stare at an unmoving object, your brain thinks the object is moving in the opposite direction.
Credit: R. Adams, found this illusion in the early 19th century.