4 - 4 = ？

One day, the teacher inquired of 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.

四减四等于几？
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！你想，我要是往你口袋里放四个硬币，而你的口袋上有个窟窿，硬币全漏掉了，那么，你衣袋里还剩下什么？”
“窟窿，”彼得答道。