One day, the teacher inquired Peter: How much is four minus four?

Peter was tongue-tied.

the teacher got angry and said: What a fool! You see, if I put four coins in your pocket, but there is a hole in your pocket and all of them leak out, now what is left in your pocket?

The hole, replied Peter.

老师生气地说：真笨！你想，如果我放四个硬币在你的口袋里，但你的口袋里有个窟窿，结果四个硬币都漏掉了。那么，你的口袋里还有什么？

窟窿，彼德答道。