Matter occupies space and has mass. In bulk, matter can exist in several different phases, according to particle density and energy density or alternatively pressure and temperature. These phases include gases, plasmas, liquids, fluids, superfluids, solids, and Bose-Einstein condensate and have their particular properties. As circumstances change, matter may change from one phase into another. These phenomena are called phase transitions, and their energetics are studied in the field of thermodynamics. In small quantities, matter can exhibit properties that are entirely different from those of bulk material. Matter constitutes the observable Universe. It can be converted to energy (see annihilation), and vice versa - can be created out of energy (see matter creation) and undergo other formations and alterations.
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