A very simple back-seat game, for use among siblings or good friends. It goes a little something like this :

You can play with anyone that's within arm's length. Stare intently out the window looking for Volkswagen Beetles. Once you see one, be the first to call out 'Slug Bug!'. If you were the first to call it, you get to thump everyone else involved in the arm. As hard as you can. Continue on until people are so hurt that they cry Uncle. The winner is the last man standing.

Variant rules :

There's really no strategy you can use in this game, except to maybe do lots of push-ups to increase the force of your punch, and to make sure to hit your opponents in the same spot every time - create a weakness, and exploit it.

My favorite way to play is to get a window seat on an airplane, and play with your neighbor in the aisle seat - unless if someone smuggles a Volkswagen under their skirt ('Um... I'm pregnant. Yeah, that's the ticket...') you will have very smooth sailing.