Mathematics
The Pigeonhole Principle with Surprising Examples
Quick fact
In any group of 367 people, at least two share a birthday, because there are only 366 possible birthdays (including February 29).
Why this is interesting
Imagine you have 10 pairs of socks but only 9 drawers. You will definitely have one drawer with two pairs, but how can this trivial idea prove that two people in London have exactly the same number of hairs on their head?