Why do some things float while others sink? Scientists use an idea called density. Density is a measure of how much mass is packed into a certain amount of space. That space is called its volume. A thing floats if its average density is less than the density of the fluid around it. As of November 2026, a cubic centimeter of fresh water holds about 1 gram. A milliliter holds the same.

Let's start with clay, since it is easy to shape. Drop a solid lump of clay or iron in water and it sinks. But clay that sinks as a lump will float if you mold it into the shape of a boat. A boat floats because the air inside it makes its average density lower than the density of water. A sinking thing still gets a push from the water. It seems to weigh less when it is under the water.

Let's see why size and weight are not the answer. The real answer may surprise you. Water gives a push upward to anything in it. Big size and heavy weight do not settle the question of floating. Lots of kinds of wood float. A thing with the same density as the fluid hangs in it, neither sinking nor rising. Scuba divers aim for this so they can hover.

Water is not always the same, and it can change in two ways. One way is salt, and the other way is heat. Next to fresh water, sea water is slightly denser, at about 1.02 to 1.03 grams in each cubic centimeter. In general, cold water is denser than warm water, which means that temperature changes the density of water. Now think of a ship with no cargo. It floats higher than that same ship does when it carries a load.

Scientists measure density with some handy tools, and here are two of them. A tool called a hydrometer measures how dense a liquid is. It rides low in a liquid that is less dense and high in one that is denser. Specific gravity is a number that compares a thing's density with the density of water. Under 1 means the object floats, and over 1 means it sinks. Water can even help with a lumpy rock. When a thing goes under water, it shoves aside water equal to its own volume. That lets us find the volume of a lumpy thing by using water.

Next time you have a cold drink with ice, look at how the cubes sit in the glass. Ice is a puzzle. It is a solid, yet ice floats on water. An ice cube is roughly 9 percent less dense than liquid water, so most of it sits below the water line. In ice, water molecules sit in a spread-out pattern with a lot of space between them. That is why ice is less dense than liquid water.

Here are some more examples from the world around us. Nature gives plenty of cases to think about. Cooking oil rides on top of water. Helium is lighter than air, and that is why a helium balloon floats in the air. Most fish have a swim bladder filled with gas, and it helps them hang in the water. Sharks are different. Sharks have no swim bladder. Their big, oily liver slows their sinking. Think about how each one stays up or sinks down.

One place has very special water. The Dead Sea is very salty and dense, so bathers stay buoyant. Very few living things can survive there. Floating can look easy, but a life jacket helps a person float. Kids, and everybody who is in a boat, should wear one. There are child sizes of life jackets because one made for adults might not work for a child. Water play is fun, and it is best when it is safe. Before you go in, always ask first, and swim only when a grown-up is watching.

Words to know

• density: mass per space
• volume: amount of space
• hydrometer: density meter
• hover: hang in place