It is designed to prove the Pythagorean theorem. Water tank: as the water or the beads in the 1st and 2nd tank is rotated, the 3rd tank is filled. In this model, the smaller squares of water construct the two shorter sides of a right triangle. When the water is allowed to flow into the larger square on the longest side, known as the hypotenuse, it demonstrates that the two squares exactly fill the lower square. This is no proof, but it is a nice demonstration of the a2 + b2 = c2 theorem.