Drag the points in the sketch below.[br]What do you notice? What does this prove? How does this prove it?

Proof by H. Perigal (1830) and H. E. Dudeney (1917)[br]See Cut the Knot: [url=http://www.cut-the-knot.org/pythagoras/index.shtml#27]Pythagorean Theorem[/url]