i teach economics at unlv three times per week.last monday, at the beginning of class, i cheerfully asked my students how their weekend had been.one young man said that his weekend had not been so good. he had his wisdom teeth removed.the young man then proceeded to ask me why i always seemed to be so cheerful.

　　我在内华达大学拉斯维加斯分校教经济学，每周上三次课。上周一，在刚开始上课的时候，我兴致勃勃地问学生们周末过得怎么样。一个男生说，他的周末不太愉快，因为他的智齿被拔掉了，结果让他痛了一整天。然后，他又问我为何我总能保持那么快乐的心情。