My middle school students like to play a game I like to call "Kai Bai Boh -- slap!"

Kai Bai Boh is the Korean name for "rock, paper, scissors" (we call it 'jon ken po' in Hawaii, though I have no clue why).
KBB is quite simple: two people play and the winner gets to hit the loser. Despite the complexity of their academics (learning excel function code and learning about asymptotes for example), they are entertained by giving and receiving physical abuse from friends.
Though KBB seems like a simple game, some have suggested that it is quite intricate in nature. Although everyone has an equal chance of winning, the psychological factors can possibly give a tremendous advantage to one party. Maybe that's how I went 0-3 with a student yesterday.

