Count the number of times the white team passes the basket ball in this video. Don't count passes by people with black shirts, just ones in white shirts. Both aerial and bounce passes should be counted.
(Note: Requires Java for a player applet. Takes a while to load; it's around 7 MB. Sorry, dial-up users, it's not my fault.)
Have you counted the passes? Good.
About half of the people will not see the gorilla.
Really, right in the middle, someone in a gorilla fursuit walks through the action. Go check! (And if you did see the gorilla, did you still count all the passes?) This video is part of a series of experiments in inattentional blindness research (PDF). This is one of many different video experiments they did.
In fact, this experiment won a 2004 Ig Nobel Prize for Daniel Simons and Christopher Chabris, who accepted it in good humor. They won in the Psychology category, basically as "best experiment to involve a gorilla costume".