<P> A keg is a small barrel . </P> <P> Traditionally, a wooden keg is made by a cooper and used to transport items such as nails, gunpowder, and a variety of liquids . </P> <P> More recently, a keg is often constructed of aluminum or steel . It is commonly used to store, transport, and serve beer . Other alcoholic or non-alcoholic drinks, carbonated or non-carbonated, may be housed in a keg as well . Such liquids are generally kept under pressure . </P> <P> Beer kegs are made of stainless steel, or less commonly, of aluminium . A keg has a single opening on one end, called a "bung ." A tube called a "spear" extends from the opening to the other end . There is a self - closing valve that is opened by the coupling fitting which is attached when the keg is tapped . There is also an opening at the top of the spear that allows gas (usually carbon dioxide) to drive the beer out of the keg . The coupling fitting has one or two valves that control the flow of beer out of and gas into the keg . The keg must be in the upright position, that is, with the opening on top, for the beer to be dispensed . Kegs can be contrasted to casks, which have two or more openings and no spear . Most major breweries now use internally speared kegs . </P>

What is a beer keg made out of