A compilation of proofs for the theorem of Pythagoras, by Ed Southall. Thanks for the suggestion, comicremixes!
Using this in class tomorrow.
For the older crowd.
Tesselating bunny snub cube.
Unrolling these circles gives fills a triangle with base 2 π r and height r (where r is the radius of the filled disk). Such a triangle has area π r2. This does not serve as a complete proof for why this is the area of a circle, but can give you some intuition for why it should be. [code]