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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.
一天，老师问Peter：“4减4等于多少？”Peter答不上来。老师很生气：“真笨！你想，如果我放4个硬币到你包里，但是你包有一个洞，硬币都掉了，现在你包里还剩什么？”“一个洞。”