Matter can exist in one of three main states: solid, liquid, or gas. A solid can transform into a liquid through melting, and a liquid can transform into a solid through freezing. A solid can also change directly into a gas through a process called sublimation. A liquid can be converted to gas through heating at constant pressure to the substance’s boiling point or through reduction of pressure at a constant temperature. This process is called evaporation. A gas at a temperature below its critical temperature can also be called a vapor. A vapor can be liquefied through compression without cooling.