Phases of matter can be changed by a number of things. The most common ones are temperature and pressure. Substances usually become a gas at warm temperatures and/or low pressures, become a solid at cool temperatures, and become a plasma at extremely hot temperatures. Substances often become liquid in between solid and gas, but when the pressure is very low (such as the vacuum of outer space) many substances skip the liquid phase, going directly from solid to gas or vice versa. Scientists have created graphs called phase change diagrams to show the relationship between pressure, temperature, and phase of many substances.