春秋时代，有一个国家叫做晋国。
晋国的君主晋献公特别喜欢一个叫骊姬的女人。
为了让她高兴，晋献公杀了自己的大儿子申生，并改立骊姬的儿子奚齐为太子。
晋献公还有两个儿子，重耳和夷吾。
他们看到父亲这样对待自己的兄弟，感到很害怕，于是逃到了别的国家。
后来，夷吾在秦穆公的帮助下回到了晋国，继承了帝位，成为了晋惠公。
可是，他并没有履行之前答应给秦国的一些土地。
更糟糕的是，在秦国发生饥荒的时候，晋惠公也没有给予帮助。
鲁僖公十五年，秦穆公决定派兵攻打晋国。
晋惠公亲自带领军队去迎战。
但他选择了一辆由郑国进贡的四匹小马拉的车。
晋国的大臣庆郑看到了，对他说：“以前打仗的时候，我们都用自己国家的马来拉车。因为这些马和我们生活在同一片土地上，它们能理解我们的心意，接受我们的训练，熟悉我们的驾驭方式。但是现在你选用了外国的马，如果它们受到惊吓，就会变得害怕，违背我们的意愿。这样一来，它们内心会变得狂乱、暴躁，血液循环加快，血管膨胀，外表看起来很强壮，但实际上内部已经虚弱不堪。到那时，你让它们前进或后退，它们都不会听从你的命令。”
但晋惠公没有听从庆郑的建议，最后在韩原被秦军抓住了。
这个故事中的“外强中干”，原本是用来形容马受惊后的心理和生理变化的，后来也被用来比喻看似强大，实则虚弱的事物。
In the Spring and Autumn Period, there was a country called Jin.
The ruler of Jin, Duke Xian, favored a woman named Li Ji very much.
To please her, Duke Xian killed his eldest son Shen Sheng and replaced him with her son Xi Qi as crown prince.
Duke Xian had two other sons, Chong'er and Yi Wu.
They were afraid when they saw their father treat their brother like this, so they fled to other countries.
Later, Yi Wu returned to Jin with the help of King Mu of Qin and inherited the throne, becoming Duke Hui of Jin.
But he did not fulfill the promise he made to give Qin some land.
Worse still, when Qin suffered from famine, Duke Hui of Jin did not provide any assistance.
In the 15th year of Duke Xi of Lu's reign, King Mu of Qin decided to send troops to attack Jin.
Duke Hui of Jin led his army in person to meet the enemy.
But he chose a carriage pulled by four small horses presented by Zheng.
When Qing Zheng, a minister of Jin, saw this, he said to him, "In the past, when we went to war, we always used our own horses to pull our chariots. Because these horses lived on the same land as us, they could understand our intentions, accept our training, and become familiar with our way of handling them. But now you have chosen foreign horses. If they get frightened, they will become scared and act against our wishes. In that case, they will become agitated and irritable inside, their blood circulation will speed up, their blood vessels will expand, and they will look strong on the outside but weak inside. At that time, if you ask them to go forward or backward, they will not obey your orders."
But Duke Hui of Jin did not listen to Qing Zheng's advice and was eventually captured by the Qin army at Han Yuan.
The phrase 'empty inside outwardly strong' in this story originally described the psychological and physiological changes of horses when frightened. Later it was also used to describe things that appear strong but are actually weak.
