7. Thumb Wrestling
The Rules: Players face each other, then link opposing hands, keeping their thumb "up." The goal is to to pin your opponent's thumb for three seconds. In the interest of fair play, a match usually begins with "1,2,3,4 I declare a thumb war" to remove any unfair element of surprise.
When you are in a thumb wrestling phase, you will fill any amount of free time with a match. These phases are usually short-lived, however, as an arms race develops between you and your competitor. Very quickly, players resort to using unfair leverage and extra fingers to gain an edge.
In pursuit of victory by any means necessary, it is so easy to forget that rules are what separate us from the animals.