Pigeonhole Principle

The Basic Version of the Pigeonhole Principleis:

"For all positive integers $n$ and $m$ with $m<n$, if $n$ objects are put into $m$ boxes, then at least one of the boxes contains at least $2$ objects."

Proof of Pigeon Hole Principle
http://www.proofwiki.org/wiki/Pigeonhole_Principle