The animation starts with one right angle triangle with hypothenuse c and legs a and b.
Then 3 copies are created and arranged within a square of side length a + b.
The arrangement leaves an area of c² uncovered within the boundaries of the square.
Then the 4 triangles are rearranged and the uncovered area is seen to be a² + b².
Since moving around the triangles does not changed the total area, we see that c² = a² + b².
Animation was created by Erich Neuwirth using MswLogo.