Mark Twain is an American writer. One day, he went to a city, the train to visit a friend in there, he is a very busy man, often forget things. When he got on the train, conductor to see his ticket. Mark Twain looked everywhere for tickets, but could not find him_ The conductor know Mark Twain, he said: "if you find nothing, in the relationship. When you come back, let me see your ticket." "Oh can't", Mark Twain said, "I must find the ticket, if I find it, how do I know where I am going?"