Cholecalciferol is the most widely known of the vitamin D series and is a fat soluble vitamin that is stored to some degree in the body. Many people know that sunlight is a source of vitamin D, but this is true only in the fact that ultraviolet light from the sun acts as a catalyst on a vitamin D precursor in the skin. Vitamin D formed in this manner is termed "natural vitamin D" or vitamin D₃. Vitamin D precursors obtained from milk and other products as well as sunlight-produced vitamin D must be metabolized in the liver to form the active coenzyme. The primary functions of vitamin D are stimulation of calcium and magnesium absorption, two minerals that are essential for strong bones. Sources of vitamin D include fish liver oil, butter, and milk.