Mr. Black has two cats. One is big and the other is small. He likes them very much. One day , his good friend , Mr. Green , comes to see him. He is very surprised( 感到惊奇的 ) to find there are two holes( 洞 ) in the door , a big one and a small one. “My dear friend , why are there two holes in your door?” “To let them in and out , ”Mr. Black answers. “But why are there two holes?” asks his friend. “How can the big cat go through the small one?”he says. “I'm sorry. I don't think too much. I just think the big hole is for the big cat and the small one is for the small cat , ”answers Mr. Black. “OK. A big hole is enough( 足够的 ). Is that so?”asks his friend. “I think so , ”says Mr. Black.