One day, the teacher inquired¹ 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.

一天，老师问彼得：“4减4等于几？”彼得张口结舌答不上来。

老师生气地说：“真笨！比如我给你衣袋里装进4个硬币，可你衣袋上有个窟窿，硬币全从这里漏掉了，那么，你衣袋里还剩下什么?”“还剩下个窟窿。”彼得答道。