Why Bubbles Fall in Guinness

Sometimes, scientists should probably not spend too much time contemplating the beer they are drinking.

A group of Irish mathematicians attempted to explain why bubbles in a pint of Guinness (naturally) fall when bubbles in a pint of lager, say, rise.

After hours of complicated math, they determined that it depends on the shape of the glass. When poured down the center of a cylindrical glass all the bubbles will rise together. But when poured down the center of a regular shaker pint glass or the ubiquitous Guinness tulip glass, the bubbles in the center rise to the top, causing a pressure imbalance which pulls the bubbles on the sides of the glass down to the bottom of glass and into the column of rising bubbles.

I think that's how it works. Maybe you could get a clearer explanation here or here.