唇亡齿寒
春秋时候，晋献公想要扩充自己的实力和地盘，就找借口说邻近的虢（guó）国经常侵犯晋国的边境，要派兵灭了虢国。可是在晋国和虢国之间隔着一个虞国，讨伐虢国必须经过虞地。“怎样才能顺利通过虞国呢？”晋献公问手下的大臣。大夫荀息说：“虞国国君是个目光短浅、贪图小利的人，只要我们送他价值连城的美玉和宝马，他不会不答应借道的。”晋献公一听有点舍不得，荀息看出了晋献公的心思，就说：“虞虢两国是唇齿相依的近邻，虢国灭了，虞国也不能独存，您的美玉宝马不过是暂时存放在虞公那里罢了。”晋献公采纳了荀息的计策。
虞国国君见到这两们珍贵的礼物，顿时心花怒放，听到荀息说要借道虞国之事时，当时就满口答应下来。虞国大夫宫之奇听说后，赶快阻止道：“不行，不行，虞国和虢国是唇齿相依的近邻，我们两个小国相互依存，有事可以自彼帮助，万一虢国灭了，我们虞国也就难保了。俗话说：‘唇亡齿寒’，没有嘴唇，牙齿也保不住啊！借道给晋国万万使不得。”虞公说：“人家晋国是大国，现在特意送来美玉宝马和咱们交朋友，难道咱们借条道路让他们走走都不行吗？”宫之奇连声叹气，知道虞国离灭亡的日子不远了，于是就带着一家老小离开了虞国。
果然，晋国军队借道虞国，消灭了虢国，随后又把亲自迎接晋军的虞公抓住，灭了虞国。 故事出自《左传·僖公五年》。成语“唇亡齿寒”，比喻双方关系密切，相互依存。
if the lips are gone, the teeth will be cold
during the spring and autumn period (707 - 476 b.c.), duke xian of the state of jin wanted to expand his position of strength and sphere of influence. therefore he would like to send his troops to destroy the state of guo on the pretest that the state of guo often encroached on theborders of the state of jin. but there was a third state, the state of yu,between the state of jin and the state of guo, and the jin army had to cross the state of yu before it could reach the state of guo . "how can my army cross the state of yu without a hitch?" duke xian of the state of jin asked his ministers.xun xi , one of the ministers, said, "the monarch of the state of yu is short - sighted and covets small advantages. if we give him priceless precious stones and fine horses, it is not unlikely that he will allow our army to pass through his country. " seeing that duke xian of the state of jin was a little bit grudging, xun xi continued to say, "the state of yu and the state of guo are neighbor stated as closely related as lips and teeth. the state of yu cannot exist independently if the state of guo is destroyed. your precious stones and fime hores are just left in the care of the monarch of the state of yu." so duke xian of the state of jin accepted xun xi's plan. when the monarch of yu saw the precious gifts, he was elated,and readily promised to let the jin army pass through his state. hearing the news, gong zhiqi, one of the ministers as the state of yu, hastened to admonish the monarch, saying," that won't do.for the state of yu and the state of guo are neighbor states as closely related as lips and teeth. our two small states are interdependent, and can help cach other when problems crop up .if the state of guo were destroyed, it would be difficult for our state of yu to continue to exist. as the common saying goes, if the lips are gone ,the teeth will be cold, the teeth can hardly be kept if the lips are gone. so it won't do at all to allow the jin army pass our state." the monarch of the state of yu said," the state of jin is a big state. now they here specially to present gifts to us with the intention of being on friendly terms with us. under suchcircumstances, how can we refuse to allow them to pass through our state?" hearing this, gong zhiqi sighed repeatedly. knowing that the state of yu would soon be destroyed, gong zhiqi left the state of yu together with his whole family.
as expected, the troops of the state of jin, allowed to pass through the state of yu, destroyed the state of guo and on their return trip captured the monarch of the state of yu who went out personally to meet them, htus destroying the state of yu as well.
this story appears in the chapter " the fifth year of duke xi " in zuo zhuan,the famous commentary by zuo qiuming on the spring and autumn annals. the set phrase " if the lips are gone, the teeth will be cold " is used to mean that two persons or things share a common lot and that is one fails ,the other is in danger.