Luiz Pereira, a physicist at the Ludwig Maximilian University of Munich, and his colleagues recently published the results of their study of why peanuts move up and down inside beer.
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Placing peanuts inside beer is apparently a custom in Argentina, so generations of drinkers have noticed that peanuts tend to move up and down repeatedly. Why? If I understand the article correctly, it is because during their descent into the beer, air bubbles within the liquid attach to the peanuts, causing them to become buoyant. When the peanuts arrive at the surface of the beer, the bubbles pop and the peanuts lose their buoyancy.
-via Dave Barry