In the last decade, many beers have come to be endowed with an especially fine, creamy head that used to be largely limited to stouts. The creamy head comes from an artificial dose of nitrogen gas that may be injected into beer at the brewery, or in the bar or pub by the tap that delivers beer from the keg, or by a small device inside an individual beer can. Nitrogen is less soluble in water than carbon dioxide, so its bubbles are slower to lose gas to the surrounding liquid, and slower to coarsen and deflate. Nitrogen bubbles remain small, and persist. They also don’t carry the tart prickliness of carbon dioxide, which becomes carbonic acid when it dissolves in beer and on the surface of our tongue.