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Mark Twain is a great, American writer. One day he goes to' a city by train. MarkTwain is always very busy, so he usually forgets (忘记) things.

When he is on a train one time, the conductor asks him for his ticket. But he can't find the ticket. The conductor knows (认识) Mark Twain, He said." Show me your ticket on your way back, And if you can't find it, it doesn't matter?

"Oh, no," says Mark Twain," I must find the ticket. If I can't find it, how will I know where I'm going?