Cool Proof

It is a mathmatical certainty that at least two people on the Earth have the exact same number of hairs on their body.

First find an acceptably larger number.  ie.. overestimate.

Nobody on Earth is 100 inches (8.3ft) high and nobody is 100 inches around.  100 * 100 = 10,000 square inches of skin.  Overestimate the number of hairs per square inch... say 10,000 hairs per square inch.  10,000 * 10,000 = 100,000,000 hairs.  Assume that we have a room for each one of those hairs... 100 million rooms.  There are 8.6 billion people on the planet.  Everybody lines up and each person enters the room that marks the exact number of hairs on their body.  Let's say that after a while, every possible room has a person in it.  We've still got 8.5 billion people waiting to enter rooms... we are forced to allow them to double up... hence we have proven the theory.