Here's why and science has the answer:
"The mystery of why the bubbles in the legendary beer Guinness sink as opposed to rise as one might expect has finally been solved the secret apparently lies in the shape of the pint glasses from which Guinness is often sipped, researchers in Ireland say. After you pour a glass of the famous Irish stout Guinness, the white bubbles settle downward. Since bubbles are lighter than beer, you might think this defies the laws of gravity.
"In one's everyday life, one rarely comes across such a counterintuitive phenomenon, challenging equally the imagination of a university professor as well as that of Bill, John and Harry from the local pub," said researcher Eugene Benilov, an applied mathematician at the University of Limerick in Ireland.
The solution to this puzzle lies in how the beer flows in the glass. The beer flows downward near the walls of the glass, dragging the tiny bubbles along with it, and then upward in the interior. This circulatory pattern eventually leads to a creamy white head of foam resting on top of the almost-black brew."
More here: http://www.msnbc.msn.com/id/47681338...ience-science/
And I thought it had more to do with how much you drink!