021: 1/3 cube

Three copies of this pyramid fit together to form a cube. Together with Cavalieri's principle this nice dissection shows that the volume of a pyramid/cone is 1/3 x base x height. (B.t.w., in a way this pyramid is also a "continuous" counterpart to the Square Tower, one of the other models in this collection. Three square towers almost fit together into a cube, which shows that the sum of the first n squares is approximately n^3/3.)