一位教授在某大学毕业典礼上演讲。他问这些即将走向社会的年轻人:“有多少人锯过木头?请举手。”大多数学生都举起了手。接着,他又问道:“有多少人锯过木屑?”全场哑然,没有一个人举手。“当然,你们不可能去锯木屑,”教授说,“因为木屑是已经锯下来的木头。过去了的事情就是木 A professor speaks at a college graduation ceremony. He asked these young people who are about to reach the society: “How many people saw wood? Please raise your hand.” Most students raised their hands. Then he asked again: “How many people saw sawdust?” The audience was dumb, and no one raised their hands. “Of course, you can’t go to sawdust,” said the professor, "because sawdust is already sawn wood. The thing that passed was wood.