During the Spring and Autumn Period (770–476 BC), Duke Xian of the State of Jin sought to expand his power and sphere of influence. He planned to attack the State of Guo, claiming it had encroached on Jin's borders. However, the State of Yu lay between Jin and Guo, forcing the Jin army to pass through Yu.
"How can my army cross Yu without a hitch?" Duke Xian asked his ministers. Minister Xun Xi replied, "The ruler of Yu is short-sighted and covets small gains. If we offer him precious stones and fine horses, he will likely grant us passage."
Seeing Duke Xian's reluctance, Xun Xi continued, "Yu and Guo are as closely related as lips and teeth. If Guo is destroyed, Yu cannot survive alone. Your treasures are merely being stored with the ruler of Yu for now."
Duke Xian agreed. The ruler of Yu, delighted with the gifts, readily promised passage. Upon hearing this, Minister Gong Zhiqi of Yu urgently admonished his ruler: "This will not do. Yu and Guo are interdependent like lips and teeth. If Guo falls, Yu will be in peril. As the saying goes, 'If the lips are gone, the teeth will be cold.' We must not allow the Jin army through."
The ruler of Yu said, "Jin is a major state offering us gifts in friendship. How can we refuse them?" Gong Zhiqi sighed repeatedly and, foreseeing Yu's doom, left with his family.
As predicted, the Jin army passed through Yu, destroyed Guo, and on their return captured the welcoming ruler of Yu, destroying Yu as well.
This story from Zuo Qiuming's commentary Zuo Zhuan ("The Fifth Year of Duke Xi") gives rise to the idiom "If the lips are gone, the teeth will be cold," meaning that interdependent parties share a common fate; if one falls, the other is endangered.