A man of the state of Chu (chǔ guó 楚国) had a spear and a shield for sale. He was loud in praises of his shield.
"My shield is so strong that nothing can pierce it through."
He also sang praises of his spear.
"My spear is so strong that it can pierce through anything."
"What would happen," he was asked, "if your spear is used to pierce your shield?"
It is impossible for an impenetrable shield to coexist with a spear that finds nothing impenetrable.

In ancient times there was a man who played the zither very well. Once, he played a tune in front of a cow, hoping that the cow would apperciate it. The tune was melodious, but the cow showed no reaction, and just kept on eating grass, The man sighed, and went away.
This idiom is used to indicate reasoning with stubborn people or talking to the wrong audience.

In AD 383, the king of Former Qin (qín cháo 秦朝), Fu Jian (fú jiān 苻坚), led a huge army to attack Eastern Jin. After losing the first round of fighting, Fu Jian looked down from a city wall, and was terrified when he saw the formidable battle array of the Eastern Jin army. And then looking at the mountains around, he mistook the grass and trees for enemy soldiers. As a result, when the nervous Fu Jian led his army into battle, it suffered a crushing defeat.
This idiom describes how one can defeat oneself by imaging difficulties.

东晋时代，秦王苻坚控制了北部中国。公元383年，苻坚率领步兵、骑兵90万，攻打江南的晋朝。在淝水一带，被晋军出奇击败，损失惨重，大将被杀，士兵死伤万余。秦军的锐气大挫，军心动摇，士兵惊恐万状，纷纷逃跑。此时，苻坚在寿春城上望见晋军队伍严整，士气高昂，再北望八公山，只见山上一草一木都像晋军的士兵一样，于是更加恐慌。后来苻坚全军覆没，带着少数残余的队伍逃回去了。
这个成语后来形容人在极度惊恐时，疑神疑鬼的心理状态。

Ban Chao (bān chāo 班超) was a famous diplomat and military figure of the Eastern Han Dynasty (dōng hàn 东汉). In 73 AD, Ban Chao and 36 subordinates were on a diplomatic mission to the kingdoms of the Tarim Basin, and first arrived at the kingdom of Kroran, also known as Loulan (lóu lán 楼兰) or Shanshan (shàn shàn 鄯善).

In the Warring States Period (zhàn guó 战国, 475-221 BC), Lian Po and Lin Xiangru were both officials of the State of Zhao. Having rendered outstanding service, Lin Xiangru was promoted above Lian Po. Disgrunted, Lian Po announced that he intended to humiliated Lin the next time he met him. Lin Xiangru, putting the benefit of the country first, avoided Lian Po in order not to cause conflict, thus bringing opprobrium on himself. Later when Lian Po realised his mistake, he was so ashamed that he went to Lin’s home carrying brambles on his naked back and asking for punishment.
This idiom indicates acknowledging one’s mistake and offering a sincere apology.