If you tie 13 knots, all the same distance apart, in a piece of string, and make a triangle with knots at the corners and three gaps between knots on one side, four on the next and five on the third, you will have a right-angled triangle.

This works because of Pythagoras’ theorem – the square on the hypotenuse (the sloping side) is equal to the sum of the squares on the other two sides.