Mercury is a naturally occurring metal found throughout the environment. It exists in three forms: metallic or elemental mercury; inorganic mercury; and organic mercury. In its metallic form, it is the familiar shiny, silver-white odorless liquid used in thermometers. At room temperature, it can evaporate to form mercury vapor. When mercury combines with other elements such as chlorine, sulfur, and oxygen, it forms inorganic mercury or mercury salts. Organic mercury is formed when mercury combines with carbon. The most common organic mercury compound is methylmercury. Mercury enters the environment from the breakdown of rocks and soil, from volcanic activity, and from human activity such as mining and burning coal.