Third Guy Ducks

Listen here, buddy. So, like, two dudes are just strollin' into a bar, right? But then there's this THIRD guy who's all like, "Gotta dodge the bar roof!" It's funny 'cause normal people don't go around ducking when they enter a building. The joke is funny because it presents an unexpected or absurd situation. It plays on the listener's expectations by setting up a typical scenario (people walking into a bar) and then subverting it with an unusual action (one person ducking).