As mentioned in "Zhuangzi", the secret recipe of Song State was sold to Wu State by merchants. Wu State relied on this secret recipe to defeat Yue.
A family in the Song Dynasty could produce a medicine to prevent and treat frostbite on the skin. Therefore, this family has been rinsing cloth in water for generations. Later, a very shrewd businessman heard the news and invested hundreds of dollars to buy their secret recipe. This person took this "patent" and came to the southern state of Wu. He knew that Wu and Yue were at war at that time. If the Wu army officers used this medicine, they would not suffer from frostbite during the water war, so he dedicated this secret recipe to Wu. king. After King Wu got the medicine, he felt like he had found a treasure and launched an attack on the Vietnamese army in the severe cold winter. Although the Vietnamese army resisted vigorously, their combat effectiveness was greatly reduced due to frostbite on their soldiers, and they were finally defeated. Later, the King of Wu granted the businessman a large piece of land, which made the small businessman rich and rivaled the prince. However, the Song family that originally invented this secret recipe could only make a living by washing cloth for others.