Politeness is a symbol of human civilization, which also is an important criterion of human social activities. As a kind of social activities, activities of language are also restrained by this yard stick. In most cases, communication is decided by people's social relations. Meanwhile, a language behavior must be a social behavior. People must consider the various factors which can be accepted in society when they are communicating. We should use language appropriately to avoid unhappiness and misunderstanding in communication. So it is important for us to learn how to use polite words.

The purpose of this paper is to inform readers what humor is and how humor works in the American sitcom. That will help Chinese audience better understand the American sitcoms. In this paper, some humorous conversations will be used to show the author’s points. These humorous conversations are against the rules of politeness principle by Leech. 从语用学角度分析美国情景喜剧《破产姐妹》会话幽默(3):http://www.751com.cn/yingyu/lunwen_27068.html