Buoyancy, the tendency of an object to rise or float when immersed in a fluid (either a liquid or a gas). Any fluid exerts an upward force on an object immersed in it. The strength of the force is equal to the weight of the fluid displaced (pushed aside) by the object; this principle is known as Archimedes' Principle. This principle explains why less effort is required to lift an object submerged in water than to lift the same object when it is out of water.

If a wooden board is placed in a bathtub full of water, some of the water will spill over the side. The amount of water pushed aside by the board is called the board's displacement. The board sinks only until it has displaced a volume of water whose weight equals the weight of the board. A metal ship can float because its hull displaces an amount of water whose weight is greater than the weight of the ship. If a ship fills with water, however, the combined weight of the ship and water will exceed the buoyant force, and the ship will sink. When the density (the ratio of the mass to volume) of an object matches the density of the surrounding fluid, it will neither rise nor sink, but will remain suspended in the fluid.