The principle of Archimedes

The force of Archimedes also called buoyancy is an upwards acting force that any body plunged into a fluid is subjected to. It is the resultant of the pressure forces exerted on the body by the surrounding fluid. It is equal and opposite to the weight of the displaced fluid. So if the weight of the body is smaller than the weight of the displaced fluid, or equivalently if the mean density of the body is smaller than the density of the fluid, the Archimede force dominates and the object floats, otherwise it sinks.