Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grains—most commonly malted barley​,​ although wheat​,​ maize (corn)​,​ rice​,​ and oats are also used. The fermentation of the starch sugars in the wort produces ethanol and carbonation in the beer. Beer is one of the oldest alcoholic drinks in the world​,​ the most widely consumed​,​ and the third most popular drink after water and tea. Most modern beer is brewed with hops​,​ which add bitterness and other flavours and act as a natural preservative and stabilising agent. Other flavouring agents​,​ such as gruit​,​ herbs​,​ or fruits​,​ may be included or used instead of hops. In commercial brewing​,​ natural carbonation is often replaced with forced carbonation. Some of the earliest writings refer to the production and distribution of beer: the Code of Hammurabi included laws regulating it​,​[8] and "The Hymn to Ninkasi"​,​ a prayer to the Mesopotamian goddess of beer​,​ a recipe for it.