If you look closely at a glass of freshly poured Guiness (or any beer--but it's easier to see in a dark ale), you'll notice that some of the bubbles are actually falling instead of rising. Explain how this is possible.

(In reply to Solution by Penny)

I think that Brownian motion works on a very much smaller scale.

I guess that the bubbles travel upward and create an upward beer current.  When the beer is at the top of the glass, there is a sort of "beer vacuum" at the bottom of the glass.  The beer that travels in the direction of this vacuum may take some bubbles with it, creating falling bubbles

Posted by Hugo on 2005-01-12 14:25:49