Cows emit a massive amount (about 280 liters or 74 gallons) of methane through belching, with a lesser amount through flatulence. This amount is comparable to the pollution produced by a car in a day.
Cows belong to a class of animals called ruminants. Ruminants have four stomachs and digest their food in their stomachs instead of in their intestines, as humans do. Ruminants eat food, regurgitate it as cud and eat it again. The stomachs are filled with bacteria that aid in digestion, but also produce methane.