Ah, a² + b² = c². The good 'ol Pythagorean Theorem.
There are countless ways to prove -- think geometry class proofs -- the Pythagorean Theorem. One book alone, The Pythagorean Proposition by Elisha Scott Loomis, contains 256 of them. But if Euclid's proof or using differentials (a.k.a. calculus) or algebra to solve it brings a sweat to your brow, ditch your pencil but keep your paper and try this origami-esqe version: