What is a funny joke about a penguin?

Dark humor, but definitely not risque: A man walks into a bar and tells the bartender, "Gimme a shot of whiskey." The bartender gives him one, and he drinks it. He asks the bartender, "Bartender, how tall is a penguin?" The bartender says, "Oh, about three feet tall." The man says, "Give me another shot." The bartender pours him another shot and he drinks it. Then he asks the bartender AGAIN, "Bartender, how tall is a penguin." The bartender repeats his earlier answer, "Oh, about three feet tall." So the man says, "Gimme another shot." The bartender gives him the whiskey and he drinks it. Then he asks the bartender for a THIRD time, "Bartender, how tall did you say penguins were?" The bartender replies, "I just told you. About three feet tall." The man grimaces, and covers his face, crying out "Oh, no! I must have run over a nun!"