Buoyancy is the ability of an object to float in a liquid, such as water. This concept helps to explain why some things float while other objects sink. Buoyancy is an important factor in the design of many objects and in a number of water-based activities, such as boating or scuba diving.

The mathematician Archimedes discovered much of how buoyancy works almost 2000 years ago. In his research, Archimedes discovered that an object is buoyed up by a force equal to the weight of the water displaced by the object. In other words, a inflatable boat that displaces 100 pounds (45 kilograms) of water is buoyed up by that same weight of support. An object that floats in the water is known as being positively buoyant. An object that sinks to the bottom is negatively buoyant, while an object that hovers at the same level in the water is neutrally-buoyant.

This same idea helps to determine what will float in water and what will sink. If an object weighs more than the weight of the water it displaces, it will sink. If the object weighs less, it will float. This helps explain why a heavy ship can easily float in the water, while a much smaller and lighter brick will sink quickly. It isn't the size or shape of an object that primarily determines buoyancy, but the relation between an object's weight compared to the weight of the water the object displaces.

Buoyancy is important in a surprising number of fields. Designers and engineers must design boats, ships and seaplanes in a way that ensures that they remain afloat. In the case of submarines, experts developed ways to make them sink and bring them back to the surface. Many objects were developed with buoyancy in mind, such as life preservers and pontoons. Buoyancy affects many more things than most people imagine.

Additionally, buoyancy is very important in a number of water-related sports. Many swimmers know that there are easy ways to float at the surface, such as laying on a person's back or holding a full breath. Buoyancy becomes noticeable when a swimmer tries to dive to the bottom of the pool, which can take effort. Scuba divers work with many buoyancy issues, as divers must know how to float, hover and sink in the water. In fact, scuba divers often wear extra lead weights to counteract the positive buoyancy of their bodies and gear.