I'm a little late in realizing it, but today is Pigeonhole Day. Festivities include thinking about awesome applications of the Pigeonhole Principle. So let's come up with some. As always with these kinds of questions, please only post one answer per post so that it's easy for people to vote on them.
Allow me to start with an example:
By the way, if you happen to be in Evans at Berkeley today, come play musical chairs at tea!