You probably learned in school about the three states of matter: solids, liquids and gases. For example, H2O can be frozen into solid ice, it can be liquid water or it can evaporate to form a gas. But there is a fourth state of matter -- plasma -- and it's the most common form of matter in the universe. Plasma is formed by superheating a gas. When this happens, the electrons break free of the atoms and become free-floating, and the gas becomes positively charged (ionized). This mix of ionized gas and free-floating electrons makes up plasma. The sun and other stars are made of plasma, while it's not too common on Earth, where solids, liquids and gas rule the day.
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