††††An object seems to get lighter if it is lowered into water. This is because the water pushes against the object, supporting some of its weight. This supporting force is called the upthrust. Upthrust is equal to the weight of the fluid an object displaces or pushes away. An object will float if the weight of fluid it displaces is equal to its weight. It will sink if its weight is greater than the weight of fluid it displaces. This displacement theory is often called Archimedes' Principle.
It is said that Archimedes, a Greek inventor who lived in the 3rd century BC, discovered this principle after noticing that his bath overflowed when he got into it. It is said that he ran through the streets shouting "Eureka!" ("I have found it!"). With his principle he was able to prove that the King's goldsmith had tried to cheat him by putting silver into a gold crown.
