Archimedes, a Greek doctor, inventor, and mathematician, made the science of "hydrostatics", the study of liquids and pressure. The Archimedes principle says that the force holding up an object that is inside a liquid in part or totally. So, a ship actually does sink, until it moves an amount of water that is the same weight of the weight of the ship. The weight must be over a wide area to work. This explains why ships and boats (including submarines) float. An object's weight pulls down in the direction of gravity, but the water's buoyant (floating) force pushes it up. Because these forces counteract each other, the object seems to lose weight. NASA uses this principle to prepare their astronauts for the weightlessness of space, so they had the astronauts work on parts of the space ship inside water tanks 25 feet (7.6 m) deep.