Water can be ice. Ice is a solid. Water can be a liquid. It can be steam. Steam is a gas.

On Earth, matter is often solid, liquid or gas. All three are made of tiny particles.

In a solid, the tiny particles are packed tight. A solid, like an ice cube, keeps its shape.

In a liquid, the particles are close together. They can slide past one another. A liquid flows. It takes the shape of its container.

In a gas, the particles are far apart. They move around on their own. A gas spreads out. It fills the whole container.

Air is a gas, and a gas is matter. Gases take up space. That is why a balloon puffs up when it is filled.

Melting is a solid changing into a liquid. Freezing is a liquid changing into a solid. Ice floats on liquid water.

Heat liquid water enough, and it turns into a gas. The gas is called steam. Liquid water can also change into water vapor.

Look at a glass of ice water. Drops of water form on the outside. This is because water vapor in the air gets cold.

Dry ice is frozen carbon dioxide. It turns straight into a gas. It does not melt.

Words to know

• particles: tiny bits
• solid: keeps its shape
• liquid: flows
• gas: spreads out