《Sleep on the brushwood》
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 became the new king of the Kingdom of Wu, and he vowed to avenge his father.
He drilled his army rigidly until it was a perfect fighting force.
Three years later, he launched a war against the state of Yue, seized Goujian, king of Yue, and brought him back 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.
A few years later, Gou Jian was released to Vietnam. He immediately began to gather an army in secret.
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.