Mathematics
The Pigeonhole Principle and Its Clever Applications
Quick fact
The pigeonhole principle was first formally stated by German mathematician Gustav Lejeune Dirichlet in 1834, although the idea was known earlier. It is also called Dirichlet's box principle.
Why this is interesting
If you have 10 pairs of socks but only 9 drawers, you must put at least two pairs in one drawer—but what if that simple idea could prove that two people in London share the exact same number of hairs on their head?