Why do some things float? Let us start with a big word, buoyancy. Buoyancy means a thing tends to rise or float in a fluid. A fluid is any liquid or gas. A balloon full of helium floats in air, since helium is lighter than air. So floating is not only for water.

Next comes a word for how packed a thing is. That word is density. It is a big idea. It tells how much mass is squeezed into some space. A thing floats when its average density falls below that of the fluid around it. Being big or heavy does not decide it.

Scientists put numbers on it. Numbers let them compare things with each other. As of November 2026, the density of fresh water is about 1 gram for each cubic centimeter. That is the same as 1 gram for each milliliter. Keep that number in mind, because it is the one to compare with. A thing with an average density below it floats in fresh water.

Here is the trick. Clay and iron, as solid lumps, sink in water. A boat of heavy material can still float, though. It floats if the inside is hollow and holds air, rather than being solid. Count the air inside it, and the boat's average density is lower than that of water.

Now think about ice cubes. They are a good thing to watch. Ice floats on water. Its density is about 9 percent lower than that of liquid water. So an ice cube floats with most of its body below the water line. The rule from the last page works here too, as you can see.

Here is one more case. What if a thing matches the fluid's density exactly? Then it hangs in place, with no sinking and no rising. Scuba divers try for this when they want to hover. Fish have help too. Most of them have a bag of gas inside, called a swim bladder, that lets them hang in the water. A submarine takes in water to fill its ballast tanks when it dives.

Here is a surprise. Think about it for a moment. A thing that sinks still feels the water pushing it up, so it seems lighter under water. Water pushes up on whatever is in it. The upward push on a boat's bottom keeps the boat afloat.

Now for the part that keeps you safe. Life jackets help you float. Everyone aboard a boat, kids included, should have one on. A jacket sized for adults might not do the job on a child. Do not go in the water until you have asked and a grown-up is watching.

Words to know

• buoyancy: the push up
• fluid: liquid or gas
• density: how packed
• ballast: water tanks

The short version

A thing floats if its average density is less than the density of the fluid around it. As of November 2026, fresh water has a density of about 1 gram per cubic centimeter. A boat floats because its average density, with all the air inside, is less than the density of water.

Quick facts

• Buoyancy is the tendency of an object to float or rise in a fluid, which can be a liquid or a gas.
• As of November 2026, fresh water has a density of about 1 gram per cubic centimeter.
• Ice is about 9 percent less dense than liquid water, so most of an ice cube sits under the water line.
• An object with the same density as the fluid hangs in it, neither sinking nor rising.