In case you never had a break at school, rock, paper, scissors (RPS) is a playground game as popular in the US and UK as it is in Japan (where it dates to the 18nth century at least). On a signal, two players simultaneously make one of three hand signs, chosen at will: rock (a fist); paper (an open hand, facedown, with fingers together); or scissors (a partial fist with the index and middle finger extended). An easily memorised rule determines the winner: “Rock breaks scissors, scissors cuts paper, paper covers rock.” In other words, a player who chooses rock beats one who chooses scissors; scissors in turn beats paper; paper beats rock. This yields a winner whenever the two players choose differently. When they choose the same sign, it’s a tie.