Brewer Jason Hunter has helped his Massachusetts-based Berkshire Brewing make nitro versions of its Coffeehouse Porter, Russian Imperial Stout, Shabadoo Black & Tan, Lost Sailor IPA and Holidale barleywine and found that the science behind carbonation makes most beers seem like more of a mouthful than their alcohol content would suggest. "What happens when you drink a beer that has carbon dioxide in it is that when carbon dioxide warms up, it wants to break out of solution," Hunter says. When you drink that beer, you're warming it up and the gas is releasing in your stomach to give you a bloated feeling. Physically it feels like it's heavier, and when the carbon dioxide bubbles flow over your palate, if you look at them under a microscope they're jagged and scratching and irritating and give the perception of more heft to the beer." When nitrogen enters the mix, however, a brewer ratchets down the amount of carbon dioxide to compensate. Will Myers, brewmaster at Cambridge Brewing in Cambridge, Mass., makes several stouts with nitrogen and has done so for more than 15 years. When making beers such as Cambridge Brewing's current I Am Nitrogen dry stout, he ferments the beer still and carbonates it with a mixture that's 70% nitrogen and 30% carbon dioxide. When such beers are ready to pour, Berkshire Brewing's Hunter says a similarly blended "beer gas" is used to push it through the lines before sending it through a special tap first used by Guinness that features a restrictor plate with five to six holes poked through it and a narrow nozzle. "The nitrogen bubbles are clinging onto the beer, but they're not activated," Guinness' Murray says. "You could apply an energy to activate them, so the energy applied from a keg point of view is the velocity and friction when you push a beer down a beer line, through a restrictor plate with five holes in it and forces the beer through that." As a result, nitrogen bubbles are forced down the side of the glass through friction, only to bubble up again through the center. The surface tension at the top of the glass is stronger as a result, preventing the tiny bubbles from escaping or swelling up to fisheye size. With nowhere to go, the bubbles form an 18- to 20-millimeter head that can float two to three millimeters above the rim without spilling over and stays put for the life of the pint -- leaving rings of lacing down the side of the glass as it goes. The beer itself hasn't changed a bit, but the texture and aesthetics of it become the muse of pub poetry.