Mathematics
The Pigeonhole Principle and Its Surprising Consequences
Quick fact
If you have more pigeons than pigeonholes, at least one hole must contain more than one pigeon. For example, among any 13 people, at least two share the same birth month.
Why this is interesting
Have you ever wondered why in a room of just 367 people, you're guaranteed to find two who share a birthday? The answer lies in a deceptively simple counting principle.