Evaporation is the process where a liquid's molecules become a gas usually through heating. Evaporation can happen when liquids are cold or when they are warm, but it happens more often with heated liquids, because when matter is hotter, its molecules have more energy. When the energy in specific molecules reaches a certain level, they can switch between one form and another, e.g. from liquid to gas, and so undergo a phase change. In cooking, evaporation is normally witnessed when boiling water or a liquid sauce and the water evaporates off as steam and the liquid content reduces over time.