Pythagoras theorem is the ever famous concept of Euclidean geometry. According to it, the sum of hypotenuse equates the sum of remaining two sides. In case you want a real life proof, then here it is. The girl pours water in two of the three containers. She then shifts it to the longest side. In this way, the hypotenuse fills. The amount of water in hypotenuse is the same as the sum of base and height.