Matter is anything that has mass and takes up space, made of tiny particles called atoms. It exists mainly in three states—solids, liquids, and gases—each with different properties. Solids have a fixed shape and volume, liquids have a fixed volume but take the shape of their container, and gases have no fixed shape or volume. Matter has measurable properties like mass, volume, and density. Changes of state include melting, freezing, evaporation, condensation, sublimation, and deposition. Some things, like light, heat, sound, and ideas, are not matter. Understanding these properties and changes helps us identify, classify, and use matter in daily life.