well, we were talking about light waves and discussing how electromagnetic waves (light) carry energy. well, in the diagram that follows is an instrument that, when placed in a vaccuum, when you shine light on the white faces (these have black faces on the opposing side) of the pieces which are attatched to the disk, it will rotate around the axis (shown as the triangle). this is because momentum is associated with light waves and, long story short, because black absorbes light and white reflects it, the momentum of the light waves pushes on the white faces causing the toy (such as in the figure) to rotate counterclockwise. if it is not in a vacuum, the black absorbs the waves and the air molecules heat up infront of the black faces. this causes those molecules to bounce into the black faces and the toy would rotate clockwise instead of counterclockwise because the bouncing air molecules have more momentum than simply the light... interesting!