I love optical illusions and they normally work on me but in the following one you are supposed to be able to see a red pill and a blue pill when in fact they are both grey. I see them both as grey though - does anyone else see red and blue?
This is one of my all time favorites.
If you want some fun, print this one out twice. Cut out square A and then slide it from A to B on the other printout. You see it amazingly change colour as you slide it!
Print it out from here: