卧薪尝胆 (wò xīn cháng dǎn)
During the Spring and Autumn period （770－476BC）, the State of Wu launched an attack against the State of Yue. The King of Wu was seriously wounded and soon died. His son Fu Chai became the new King. Fu was determined to get revenge. He drilled his army rigidly until it was a perfect fighting force. Three years later, he led his army against the State of Yue and caught its king Gou Jian. Fu took him to the State of Wu. In order to avenge his father’s death, Fu let him live in a shabby stone house by his father’s tomb and ordered him to raise horses for him. Gou pretended to be loyal to Fu but he never forgot his humiliation. Many years later, he was set free. Gou secretly accumulated a military force after he went back to his own state. In order to make himself tougher he slept on firewood and ate a gall-bladder before having dinner and going to bed every night. At the same time he administered his state carefully, developing agriculture and educating the people. After a few years, his country became strong. Then Gou seized a favorable opportunity to wipe out the State of Wu. Later, people use it to describe one who endures self-imposed hardships to strengthen one’s resolve to realize one’s ambition.