Here is a story of a horse and a farmer: One day, the farmer attaches a heavy cart to the horse and demands that the horse pull the cart. “Well,” says the horse, “I cannot pull the cart, because, according to Newton’s third law, if I apply a force to the cart, the cart will apply an equal and opposite force on me. The net result will be that I cannot pull the cart, since all the forces will cancel. Therefore, it is impossible for me to pull this cart.” The farmer was very upset! What could he say to persuade the horse to move?