The Differential in this exhibit is like the differential gear used in rear-wheel-drive cars. It shows that the two rear wheels can turn independently. In this demonstration, either wheel can be stopped while the other continues to turn, but both can’t be stopped at the same time. In a rear-wheel-drive car, the power of the engine is used to make the two rear wheels spin. But when a car turns the corner, the inner wheel turns more slowly, since it covers a smaller distance than the outer wheel. Without a differential, the two wheels would be locked in step with by the turning axle, and one of the wheels would have to skid as the car went around the corner. Differentials are not needed in front-wheel-drive cars because the wheels can be pointed in the direction of the turn.