南宋寧宗開禧二年（西元1206年）宋朝派兵攻打金人，接連吃了好幾場敗仗，宋朝將士畢再遇決定撤軍，不過要如何全身而退可把他考倒了。後來畢再遇想到一個辦法，他下令三件事，第一、軍旗不可以拔起來，第二、帳蓬不能拆掉，第三、打鼓聲不能停止。等到沒有月亮的夜裡，畢再遇就下令全軍撤退，到了第二天天亮時，金兵看宋營裡，軍旗飄揚著，鼓聲也沒有停過，以為一切正常，這樣的情形過了兩三天後，金兵開始覺得有些奇怪，因為沒有看到有任何士兵走動，於是就派人前往偵察，這才發現宋營早就空無一人。可是為什麼鼓聲還響個不停，金兵走到營區後面一看，原來，畢再遇派士兵抓了數十隻羊，把牠們後腿倒吊綁在樹上，前腿剛好放在鼓上面，由於倒掛很不舒服，羊掙脫時自然會用前腿用力蹬著，鼓就被敲得咚咚作響。
三十六計之第二十一計：金蟬脫殼
原指寒蟬在蛻變時，會脫殼飛走並且將殼留在原地。後引申為運用計謀，轉移敵人的注意，再趁機脫逃的意思。
In 1206 AD during the Song Dynasty, the emperor sent an army to attack Jin but was defeated several time. Bi Zai Yu, the general of the Song army, decided to retreat. But the question of how to retreat safely troubled him greatly.
Bi Zai Yu later came up with a solution. He gave three orders: First, the flags were not to be pulled out. Second, the army tents were not to be taken down. Third, the battle drums must not stop beating. This done, Bi Zai Yu ordered his army to retreat under the cover of night. At the crack of dawn, the Jin army saw nothing unusual on the Song side: The flags were flying, and the sound of battle drums continued. It was not until a few days later that they started to become suspicious because there was no sign of movement within the Song camp. So they sent a scout to see what was going on, only to find that the Song camp had long been deserted.
But if the camp were deserted, how could the battle drums still beat? The Jin soldiers walked to the back of the camp and discovered the answer. Bi Zai Yu had ordered his soldiers to catch several goats and tie up their hind legs up on a tree with a rope. The front legs of the goats were left dangling just low enough to beat on the drums. Due to the discomfort of hanging upside down, the goats struggled with their front legs to try to free themselves. This was why the drums were kept beating.
Strategy 21: Shed Off the Shell Like a Golden Cicada
When the golden cicada periodically sheds off its skin, it would fly off but leave its outer shell intact. Here, it means to divert the enemy's attention in order to escape intact.
