Combinatorial laws

Statement

Pigeonhole principle: If $n$ items are put into $m$ containers, with $n > m$, then at least one container must contain more than one item. For example, of three gloves (none of which is ambidextrous/reversible), at least two must be right-handed or at least two must be left-handed, because there are three objects but only two categories of handedness to put them into.