The pigeonhole principle states that, if we have, say, 5 pigeons and 4 holes, and all pigeons are in a hole, then there must be a hole with more than 1 pigeon in it. More generally, if we distribute $n$ items across $m$ groups, with $n \gt m$, it must be the case at least one group contains more than one item. ~~By the pigeonhole principle, if there are more women than men on earth...~~