"she was the square on the hypotenuse of a right-angled triangle and they were only the squares on the other two sides"
Pythagorean proof
Public DomainPythagorean proof - Credit: Jellyvista, Wikimedia Commons

A playful reference to Pythagoras' theorem which has been taught to generations of schoolchildren.

The theorem states that:  in a right-angled triangle, the square on the hypotenuse is equal to the sum of the squares of the other two sides (the hypotenuse being the side opposite the right-angle).

The diagram on the left may (or may not) help you understand the concept.