I have been told that somewhere there is a model of the Pythagorean Theorem built out of glass or plastic so that the squares on the sides and hypotenuse can hold a colored liquid. The liquid is then allowed to flow into the large square on the hypotenuse and demonstrates that the two squares exactly fill the lower square. This is no proof, but it is a nice demonstration. Rather than build an actual model, I created an animated gif graphic model. to show how this might look. The two pictures below show an empty model and a model with the upper squares loaded.

Click on this link for a demonstration.