In 245 BC, in order to celebrate the grand Moon Festival, King Hernon gave the goldsmith a piece of gold and asked him to make a pure gold crown. Although the finished crown was as heavy as the original gold, the king suspected that the goldsmith had adulterated it. He ordered Archimedes to verify whether the crown was pure gold, but he was not allowed to destroy the crown. This seems impossible. In the public bathroom, Archimedes noticed his arm emerging from the water. Vague thoughts flashed through his mind. He put his arm completely into the water, relaxed his whole body, and then his arm emerged from the water again. When he stood up from the tub, the water level around the tub dropped; when he sat down again, the water level in the tub rose again. As he lay in the tub, the water level grew higher and he felt lighter. When he stood up, the water level dropped and he felt heavier. It must have been the upward buoyancy of the water on his body that made him feel lighter. He put rocks and wooden blocks of about the same size into the tub at the same time and submerged them in the water. The stone sank into the water, but he felt the wood become lighter. He had to press down on the block to submerge it. This shows that buoyancy is related to the displacement of an object (volume of the object) rather than the weight of the object. How heavy an object feels in water must be related to its density (the object's mass per unit volume). Archimedes found a solution to the king's problem here. The key to the problem was density. If the crown contains other metals, its density will be different. If the weight is equal, the volume of the crown will be different. A crown and an equal weight of gold were put into water. It was found that the crown displaced more water than the gold, indicating that the crown was adulterated. More importantly, Archimedes discovered the principle of buoyancy, that is, the buoyant force of a liquid on an object is equal to the gravity of the liquid displaced by the object.