Anna has two cats. One is very big and the other is very small. She likes them very much. One day, Yangyang, a friend of her, comes to see her. She sees two holes (洞) in the door, a large one and small one. She says to Anna, "My dear friend, why are there two holes in your door?"

    "Let the cats in and out," Anna answers.

    "But why are there two holes? Isn't one hole enough (足够)?" asks her friend.

    "How can the big cat go through the small hole?" she says.