Just try and count how many times the ball is passed between the white team. You'll probably be wrong:
http://viscog.beckman.uiuc.edu/grafs/demos/15.html
DO NOT SCROLL DOWN BEFORE TRYING IT
About 50% of people actually fail to see that there is a guy in a gorilla suit walking across the screen and thumping his chest until they are told about it. If you are among them, watch the vid again and this time, look for the gorilla.