A U-Boot is a beer cocktail obtained by placing a shot of vodka into a glass of beer, typically a lager. 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.
Place a shot glass on upside down a Collins glass and fill it with vodka.
Place a pint glass over the two glasses so it sits against the shot glass and flip all.
Fill the pint glass with lager and as the drink is consumed the vodka will leak into drink.