Matter is anything that has mass and occupies space. It's the 'stuff' the universe is made of, from the tiniest atoms to the largest stars.

Matter can exist in three primary states: Solid, Liquid, and Gas.

Solids: Particles are tightly packed in a fixed arrangement and vibrate in place. Solids have a definite shape and volume. Think of an ice cube or a rock.

Liquids: Particles are close together but can move past one another. Liquids have a definite volume but take the shape of their container. Water and milk are examples.

Gases: Particles are far apart and move randomly at high speeds. Gases have no definite shape or volume; they expand to fill their container. Air and steam are examples.

Changes between these states (like melting, freezing, boiling) are physical changes, driven by changes in temperature and pressure.