King Hennon asked the goldsmith to make a pure gold crown for him. After it was done, the king suspected that the craftsman had mixed silver into the crown, but the crown was as heavy as the pure gold given to the goldsmith at the beginning. Did the craftsman play tricks? The problem of trying to test the authenticity without destroying the crown not only stumped the king, but also made the ministers look at each other. Later, the king gave it to Archimedes. Archimedes thought hard about many methods, but all failed. One day, he went to the bathhouse to take a bath. Sitting in the bathtub, he saw the water overflowing and felt his body being gently pulled up. He suddenly realized that he jumped out of the bathtub and went straight to the palace without clothes on. He shouted "Eureka" and "Frica" all the way. It turns out that if the crown is put into water, the amount of water discharged is not equal to the same weight of gold, and it must be mixed with other metals. This is the famous law of buoyancy, that is, an object immersed in a liquid is subjected to upward buoyancy, and its size is equal to the weight of the liquid discharged by the object. Later, this law was named Archimedes principle.