Mathematics
The Pigeonhole Principle and Its Surprising Applications
Quick fact
The pigeonhole principle was first formally stated by Peter Gustav Lejeune Dirichlet in 1834, and it's often called 'Dirichlet's drawer principle' or 'box principle'.
Why this is interesting
If you have more socks than drawers, you're forced to double up. So why does this simple idea help prove theorems about space, numbers, and networks?