Anything in water feels a push upward. Scientists call the way an object tends to float or rise in a fluid buoyancy. A fluid may be a liquid or a gas. Buoyancy works in gases too. Air pushes a helium balloon up much as water pushes a cork up, and helium, which is lighter than air, is what lets the balloon float.

Why does the water push up at all? Water pressure grows with depth. So the water pushes up harder on the bottom of an object than it pushes down on the top. The difference between those two pushes is the buoyant force. The upward push of the water on the bottom of a boat keeps the boat afloat. Things that sink are pushed up as well.

The push has a size, and an ancient thinker found it. The thinker was Archimedes, an ancient Greek mathematician and inventor from Syracuse, Sicily. He lived from about 287 to 212 BCE. By his principle, an object in a fluid feels an upward buoyant force equal to the weight of the fluid it pushes aside, which is also called displacing it. Compare that push with the object's weight. If the buoyant force is greater, the object rises and floats. If it is less, the object sinks. If the two are equal, the object hangs in place.

Ships show buoyancy at work. Heavy material can still make a boat that floats, if the boat is hollow and full of air rather than a solid lump. Lots of air inside is the reason a big ship floats. With no cargo, a ship rides higher in the water than the same ship carrying a load.

The water around a ship matters too. Fresh water is not as dense as salty water. In very salty water, such as the Dead Sea, people are held up so well that they float easily. In the sea, a ship floats higher than it would in fresh water, because salt water is denser.

A submarine changes its own buoyancy. It goes under by taking water into its ballast tanks. When an object matches the density of the fluid, it hangs there, not sinking and not rising. Scuba divers try to reach this state so that they can hover. Clay or iron in a solid lump sinks.

Ice gives another way to see the rule. Ice floats on water. Ice floats since liquid water is denser than it is. The test for floating is average density, which must be lower than the density of the surrounding fluid. For a boat, the average density includes all the air inside, and it is less than the density of water, so the boat floats.

Buoyancy helps people stay up, but staying safe comes first. Wearing a life jacket helps a person float. Both children and everybody riding in a boat should wear one. Child sizes exist because an adult jacket may not work for a child. Whenever you want to play or swim in water, a grown-up needs to be watching, and you need to ask first.

Words to know

• buoyancy: floating push
• displacing: pushing aside
• ballast: weight tank
• fluid: liquid or gas