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[Chad]

A U-Boot is a beer cocktail (popular in Germany, Poland, Serbia and Flanders) that is made with a glass of beer and a shot glass of vodka. In Germany, the liquor k(?)orn is sometimes used instead, while in Flanders and the Netherlands, jonge jenever is preferred.
It is called a U-Boot (German abbreviation of Unterseeboot, "submarine") because the shot glass of vodka sinks to the bottom of the glass of beer. The shot glass then "surfaces" when the cocktail is drunk.
In Flanders, it is called a "Duikboot", literally translated "Submarine". It is seen as a variant on the kopstoot (lit. "headbutt") in the Netherlands; generally speaking, the kopstoot is taken as a shot followed by beer as a chaser, although dropping the glass of jenever is not unheard-of.
In Serbia, it is called "Podmornica", literally translated "Submarine".
In southern Germany, U-Boot is usually served as a combination of Fanta and cognac instead of beer and vodka.

Source: http://en.wikipedia.org/wiki/U-Boot_(beer_cocktail)