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?
http://i.dailymail.co.uk/i/pix/2015/03/30/16/27244D7D00000578-3018215-image-a-2_1427727872594.jpg
This is one of my all time favorites.
http://en.wikipedia.org/wiki/Checker_shadow_illusion
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:
http://en.wikipedia.org/wiki/Checker_shadow_illusion#/media/File:Grey_square_optical_illusion.PNG