Both running on legs and spinning wheels shift an object’s centre of mass (a hamster’s body or BB-8′s components) forward. The hamster relies on its body weight to generate the force that acts against the ball’s inner surface, while BB-8′s internal drive system uses its mass plus a spring to apply pressure against the inside of the sphere so the wheels have traction. This is again based on Sphero’s toys, in which two large wheels spin against the sphere’s inner surface.