Head start: Scientists crack beer-froth enigma PARIS (AFP) - There is the nagging question of whether life exists other than on Earth. The enduring mystery of who made us -- and why. And then there is this: Why does the foam on a pint of lager quickly disappear but the head on a pint of Guinness linger? Answers to questions 1 and 2 are still being sought, but the Great Beer Riddle, at least, may soon be solved. Writing in the prestigious British science journal Nature, an elite scientific duo say they have devised an equation to describe beer froth. The breakthrough will not only settle the vexatious lager vs. stout debate, it will also help the quest to pour a perfect pint every time. Beer foam is a microstructure with complex interfaces. In other words: a cellular structure comprising networks of gas-filled bubbles separated by liquid. The walls of these bubbles move as a result of surface tension -- and the speed at which they move is related to the curvature of the bubbles. As a result of this movement, the bubbles merge and the structure "coarsens," meaning that the foam settles and eventually disappears. Three-dimensional equations to calculate the movement have been made by Robert MacPherson, a mathematician at the Institute for Advanced Study in Princeton, New Jersey, and David Srolovitz, a physicist at Yeshiva University, New York. They build on work by a computer pioneer, John von Neumann, who in 1952 devised an equation in two dimensions. The mathematics of beer-bubble behaviour are similar to the granular structure in metals and ceramics, so the equation also has an outlet in metallurgy and manufacturing as well as in pubs.