Counting basketballs ...
 

Watch the video here

http://viscog.beckman.illinois.edu/flashmovie/15.php

which shows a group of people, some in black T-shirts and some in white, passing basketballs to each other. Carefully count how many times those in white pass their basketball; then watch the video again, this time not counting the passes, but watching all the people involved. You will be surprised ... ;-)