Boats and other waterborne vessels operate on the principle of buoyancy, or the property of fluid that causes it to exert force at all points on an object. When objects are placed in water, they sink until they displace the equivalent of their weight in water. Boats have a hollow interior and are large enough to reach this goal without being submerged.
Smaller boats have to be made of less-dense materials such as wood or fiberglass, whereas larger boats can have an outer hull of metal that displaces enough water to hold the vessel afloat. Older ships made of wood required pitch or tar to act a sealant to prevent water from getting into the interior of the vessel.
Small boats can be propelled by oars, or paddles that push against the water and propel the boat forward. Sailboats also exist and make use of wind pressing against a large piece of canvas attached to a mast. Steering for larger vessels is performed by a rudder, or a paddle at the stern of the boat. When the rudder points in a given direction, water presses against it and drags that side of the boat, while the opposite side moves forward.