Everything you can see, touch, or breathe exists in one of the classic states of matter: solid, liquid, or gas. In a solid, particles are locked in fixed positions, vibrating in place — this gives solids a definite shape and volume. In a liquid, particles slide past each other freely while staying close together, which is why liquids have a definite volume but take the shape of their container. In a gas, particles fly apart entirely, filling any available space with no fixed shape or volume. The transitions between states — melting, freezing, boiling, condensation, sublimation — happen when energy input (or removal) overcomes the intermolecular forces holding particles in their current arrangement.
Of the 118 known elements, most are solids at room temperature (25°C, 1 atm). Only two are liquids: mercury, the silver-colored metal used in old thermometers, and bromine, a fuming reddish-brown nonmetal. Eleven elements are gases at standard conditions, including nitrogen and oxygen (which together make up 99% of the air), the noble gases, hydrogen, fluorine, and chlorine. These room-temperature states reflect each element's boiling and melting points, which in turn depend on bonding type: metallic bonds and covalent networks (like diamond) create high melting points, while weak van der Waals forces between noble gas atoms mean they remain gaseous even at very low temperatures. Helium doesn't solidify at all under normal pressure — it requires about 25 atmospheres to freeze.
Beyond the three classical states, physicists recognize a fourth state — plasma — in which gas becomes so hot that electrons are stripped from atoms, creating a soup of ions and free electrons. Plasma makes up over 99% of the visible universe: every star, including our Sun, is a ball of plasma. On Earth, plasma exists in lightning bolts, neon signs, and fusion reactors. There are also exotic states like Bose-Einstein condensates (near absolute zero) and neutron-degenerate matter (inside neutron stars), pushing the concept of "state of matter" far beyond the familiar three.