Mr. Black was a farmer. He lived alone by the forest. He had no daughters or sons. He planted rice and vegetables. Also, he sometimes caught small animals for food. So he kept two dogs to help him. One dog was big and the other was small. He liked them very much.

   One day, his friend Mr. Smith, another farmer, came to visit him. Mr. Black was very happy. They haven’t seen each other for about three years. When the friend saw two holes(洞) in the door, a big hole and a small hole, he was surprised and said, "My dear friend, why are there two holes in your door?" "Let my dogs come in and go out, of course. I finished cutting them just ten minutes ago. It took me three hours to finish the work." Mr. Smith asked again, "But why are there two holes? One is enough!" "But how can the big dog go through the small hole?" Mr. Black answered.

   Sometimes a clever man may make such mistakes.