A lump of clay sinks. Formed as a boat, it will float. Scientists compare things like these with one word. That word is density, and it tells how much mass fits into a certain space. The size of that space is its volume.

Water with salt in it is denser than fresh water. That is why the same ship floats higher in sea water than in a fresh lake. Very salty water fills the Dead Sea, and it lifts a person so well that floating is easy.

A line painted on a ship's side is called the Plimsoll line. It shows how far down the ship may safely sit. The same ship floats lower with a load than without one.

Ice floats on liquid water, because ice is less dense. Cooking oil also floats on top of the water. Neither a thing's size nor its weight decides if it floats.

Life jackets help people float. Kids and every passenger on a boat should wear one. They come in child sizes, since an adult jacket might not work well on a child. Only swim when a grown-up keeps watch, and ask first.

Words to know

• density: how packed
• volume: how much space
• mass: how much stuff
• Plimsoll: ship safety line