Flatulence consists of gases that are produced by symbiotic bacteria and yeasts living in the gastrointestinal tract of mammals and are released through the anus. Flatulence is known colloquially as farting.
The primary constituents of flatulence (collectively known as flatus) are the non-odorous gases nitrogen (ingested), oxygen (ingested), methane (produced by anaerobic microbes), carbon dioxide (produced by aerobic microbes or ingested), and hydrogen (produced by some microbes and consumed by others). Odors result from trace amounts of other constituents.
The noises commonly associated with flatulence are caused not by a flapping of the posterior, as commonly thought, but by the vibration of the anal opening. The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled.