Meaning: "To depend upon each other, to share a common lot."
Source: In Spring and Autumn with Commentary by Zuo Qiuming, it is written, "When lips are lost, the teeth feel the cold."
1.In the Spring and Autumn era, Xian Gong of the state of Jin, wanted to destroy the state of Guo, so he took some precious and beautiful jade and fine horses and brought them as offerings to the state of Yu, which bordered on Guo, asking the Duke of Yu to allow the Jin army to use Yu roads for an attack on Guo.
2.The Yu minister Gong Zhiqi advised against the deal, telling the duke, "There is a saying 'when the lips go, the teeth are cold.' The states of Yu and Guo are as interdependent as lips and teeth. When Guo is destroyed, Yu itself will soon be done for. On no account allow the Jin army passage on Yu roads!"
3.The Duke of Yu ignored this entreaty and accepted the beautiful jade and fine horses, allowing the Jin army passage on Yu roads to attack the state of Guo. Predicting subsequent disaster to Yu itself, Gong Zhiqi left Yu with his family.
4.The Jin troops used Yu for a direct attack on Guo, whose forces, caught off guard and ill-prepared, were totally routed.
5.After destroying the state of Guo, the Jin army returned the same way it had come. When the Duke of Yu came out to greet them, the army grabbed hold of the chance to arrest the duke and his high ministers.
6.The Jin forces took back their beautiful jade and fine horses, and the duke deeply regretted that he hadn't taken Gong Zhiqi's advice. But it was too late.