The PIGEON-HOLE principle (ESSAY!)
There are two non-bald men in New York city with exactly the same number of hairs on their heads!
This bizarre fact follows from a very simple and extraordinarily powerful principle first coined by Dirichlet in 1834 as the SchubfachPrinzip. Today it is known as the pigeon-hole principle. This chapter on the topic appears as chapter 20 of THINKING MATHEMATICS! Vol 2: Advanced Counting and Advanced Number Systems. The book appears here.