Cow dung, also known as cow pat, is the waste product of bovine animal species. These species include domestic cattle ("cows"), bison ("buffalo"), yak and water buffalo. Cow dung is the undigested residue of plant matter which has passed through the animal's gut. The resultant faecal matter is rich in minerals. Colour ranges from greenish to blackish, often darkening soon after exposure to air. In many parts of the developing world, and in the past in mountain regions of Europe, caked and dried cow dung is used as fuel.