Try saying this out loud. It's not only fun, it's also a concise definition. And accurate.
The Pythagorean Theorem: the sum of the squares of the lengths of the legs of a right triangle is equal to the square of the length of the hypotenuse.
This definition amuses me. I teach Pre-Algebra and Algebra; for the sake of integrating disciplines, I have my students count the prepositional phrases therein.