Two boxes of exactly the same size. Four identical right triangles go in each. Whatever space is left over has to be equal — and the left box leaves two squares while the right box leaves one. That is the whole proof of a² + b² = c². Fill the right box and watch it land.
Drag a triangle onto a dashed outline and it snaps in — or just tap one and it flies to the next outline that fits. Keyboard: Tab to a triangle, Enter to place it or take it back out. A piece only fits an outline facing the same way.