[50] And he who does not comprehend what is here said may learn to understand it from a game which is very common. Those who play with nuts are accustomed when they have placed three nuts on the floor, to place one more on the top of them producing a figure like a pyramid. Accordingly the triangle stands on the floor, arranged up to the number three, and the nut which is placed upon it makes up four in number, and in figure it produces a pyramid, being now a solid body.