In the Song Dynasty, a family made a medicine to prevent and treat skin frostbite, so this family used to wash cloth with water for generations. Later, a very shrewd businessman heard the news and invested 100 yuan to buy their secret recipe. This man came to the southern state of Wu with this "patent". He knew that wuyue and Wu Jun were at war at that time, and if soldiers used this medicine, frostbite wouldn't happen in the water war, so he dedicated this secret recipe to the king of Wu. After the king of Wu got this medicine, he got a treasure and launched an attack on the Vietnamese army in the cold winter. Although the Vietnamese army struggled to resist, its combat effectiveness was sharply reduced due to frostbite of soldiers, and it was finally defeated. Later, the king of Wu sealed a large area of land for businessmen, making a small trader rich by princes. However, the Song family who originally invented this secret recipe can only make a living by washing cloth for others.