A cry expressing one's desire to ride shotgun (i.e. in the front seat). If one passenger calls "shotgun", another may call "rock" meaning they have to roshambo (in the rock-paper-scissors sense) for the honor of the front seat. However, if one calls "shotgun, no rock" he wins by defualt, and no roshambo is necessary.

It's a juvenile game, but it prevents arguments over seating order in the car. This game is another reason it's good to be the driver; you have a guaranteed seat in the front.