All matter in the universe can exist in one of three primary states: solid, liquid, and gas. In solids, particles are tightly packed, giving them a fixed shape. Liquids have more movement in their particles, allowing them to flow and take the shape of their container, while gases have widely spaced particles that move freely and fill any space available.
Plasma is a fourth, lesser-known state of matter, which occurs at extremely high temperatures and consists of ionized particles. Changes in temperature and pressure can cause matter to transition from one state to another, a process known as phase changes, such as when ice melts into water or water evaporates into steam.