How much is four minus four? 四减四可以不等于零吗？

　　一天，老师问彼得:“4减4等于几？”彼得张口结舌答不上来。

　　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?"

　　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？”

　　"The hole." replied Peter.

　　“还剩下个窟窿。”彼得答道。