By Harold McGee
Beer is not the only intrinsically bubbly liquid we enjoy, but it’s the only one whose bubbles we expect to persist long enough to form a “head” of foam atop the glass. Beer lovers even value the ability of the foam to cling to the glass as the liquid level drops, a quality known as lacing (or, in more impressive German, Schaumhaftvermögen). There are many factors that influence foaming, from the amount of carbon dioxide dissolved in the beer to the way the beer is released from keg or can. Here are some of the most interesting.