Mr. Black has two cats. One is big and the other is small. He likes them very much. One day his friend Mr. Green comes to see him. He is very surprised(惊讶的). He finds there are two holes in the door, a big hole and a small hole.
“My dear friend, why are there two holes?" asks his friend.
“How can the big cat go through（穿过）the small hole?" he says.
