Check out a GeoGebra animation I just posted: the Pythagorean theorem.
It bears much discussion, as students are not likely to spontaneously make out its significance. One way to present it to a class is to first go through the whole thing on a projector, then go back through it slowly, discussing the meaning of each step. Finally students can be asked to write up a step-by-step explanation.
–Henri
Henri's Math Education Blog 