Proving the Pythagorean Theorem

We learned about the Pythagorean Theorem, but where did it come from? How do we know it's definitely true? What if old Pythag just made it up off the top of his mystical skull? Lucky for us, in math we can proof that things are definitely true, and there are tons of ways to prove that the Pythagorean theorem is true. Here's one of the simpler ones for you to memorize, so you can bust it out at a party and be the coolest kid in the room!