One day, Mr. Smith is going to take a train to his home town. When the train starts, one of his shoes falls (落下)on to the ground. The train is running faster and faster and he can't pick it up. Then he quickly takes off the other one and throws it on the ground near the fallen shoe. A little boy on the train asks him." Why do you do that?" "I've already(已经)lost one shoe. "Mr. Smith answers, "If a man picks them up, he may get a pair of shoes."