This is really cool!

The Pythagorean Theorem has been proved many times, and probably will be proven many more times. But only one proof was made by a United States President. Five years before James A Garfield was elected president, he came up with a proof that involves a simple sheet of paper and some scissors.

http://io9.com/james-garfield-was-the-only-u-s-president-to-prove-a-m-1037750658