A proof of the Pythagorean Theorem:
Cool, huh? If you don’t immediately see how it’s a proof of the Pythagorean Theorem… just stare at it for a while. =) You can pause the animation (and then restart it again) by clicking on it with the mouse. I made this using the programming language Processing, which is a lot like Java except it has all kinds of graphics stuff built in.
(Please bear with me if it doesn’t work, I’ve never tried embedding a Java applet in a post before… if it doesn’t work for you, let me know in a comment. And by the way, there’s a very good chance the applet won’t work if you are reading this in a newsreader or in an e-mail; to get the full applet-y goodness, visit the original post.)