Archimedes' principle states that when a body is immersed partially or entirely in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it. The principle of floatation states that, when an object floats on a liquid, the buoyant force acts on the object is equal to the weight of the object. If the weight of the object is equal to the upthrust, then the object is balanced, making the object float. Buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. When any boat displaces a weight of water equal to its weight, it floats. It is often called the "principle of flotation": A floating object displaces a weight of fluid equal to its weight. Every ship, submarine, and dirigible must be designed to displace a weight of fluid at least equal to its weight.