Matter is anything that has mass and takes up space. Solid, liquid and gas are three states of matter. We often find them on Earth. All three are made of tiny particles. Plasma is a fourth state of matter. The Sun is a ball of plasma.

In a solid, the tiny particles are packed tight. Often they sit in a regular pattern. They vibrate in place. They do not move past one another. 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 whatever container it is poured into. Liquids hardly change their size when squeezed.

In a gas, the particles are far apart. They move around on their own. A gas spreads out. It fills the whole container it is in. A gas is easy to squeeze into a smaller space.

Water can be ice, which is a solid. It can be liquid water. It can be steam, which is a gas. Water freezes into ice at 0 degrees Celsius. At normal air pressure, water boils at 100 degrees Celsius. Melting is a solid changing into a liquid. Freezing is the other way around.

Evaporation is liquid water changing into water vapor. Water vapor is a gas. Air is a mixture of gases. A cloud is not just a gas. It is a mixture of air and tiny drops or bits of water.

Words to know

• matter: stuff with mass
• particles: tiny bits
• vibrate: shake in place
• plasma: fourth state