The second gift were ball and cube with a smooth wood surface. According to Fröbel they stand at the same time for the opposite and for the identical. The child should tell them apart by the outer shape: a round, rolling body (ball) and a square, not moving body (cube). Later Fröbel added the roller to these elements, which combines functions of both bodies: the rolling of the ball, the firmness and stability of the cube. With this he expressed the thought of unity of life, which is fundamental to his philosophy and which can mainly be found in his theory of education.