Rock-paper-scissors is a game of chance (and some skill, if you have a good memory and know a little about human behavior), typically played with two people using their hands. It's also called RPS, roshambo, stone-scissors-paper (which is a more sensible name, as it encodes the primary game mechanic in a more straightforward way), and janken (the original name, as the game originated in Japan). It is commonly used for settling trivial matters, like who gets to sit in the front seat of a car.
A game of rock-paper-scissors begins with a count. Usually, players pump their hands and chant "rock-paper-scissors" (or whatever the local name of the game is, like "ro-sham-bo" or "jan-ken-pon") in order to synchronize, and either on the third or fourth count, each "throws" one of three symbols:
- Rock, a clenched fist.
- Paper, an open hand, extended with the palm down.
- Scissors, a fist with the index and middle fingers extended towards the opponent and slightly separated.
The outcome is judged according to the following mnemonic: Rock smashes scissors, scissors cut paper, and paper wraps rock.
Against a truly random opponent, rock-paper-scissors is a perfectly balanced game, and strategy is pointless. Fortunately for us, humans are not random. Computers can take into account lots of foibles of human behavior, but a good human-usable heuristic in a series of repeated games with the same person is to preferentially weight whatever symbol would have defeated the last symbol you threw, because people tend to imitate and repeat what they see -- in fact, closing your eyes can also help, because it prevents that very response.
Rock-paper-scissors-Spock-lizard (or rock-paper-scissors-lizard-Spock) is a common five-option variant of RPS. In addition to rock, paper, and scissors, the additional options are:
- Spock, represented by the Vulcan salute (open hand, palm facing the opponent, thumb extended, fingers split in a "V" between the middle and ring fingers).
- Lizard, represented by a simple mouth (think "sock puppet" or "pinch").
The mnemonic is extended: Rock smashes scissors and crushes lizard, scissors decapitates lizard and cuts paper, paper covers rock and disproves Spock, Spock smashes scissors (with his Vulcan super-strength) and vaporizes rock (with his phaser), and lizard poisons Spock and eats paper.
Lots of games use mechanics that are structured in a sort of "cycle of domination" similar to rock-paper-scissors in an attempt to generate a balanced system. These can be cycles of weapons (e.g. the "weapons triangle" of the Fire Emblem games: swords beats axes [because of their agility], axes beat spears [because of their momentum], and spears beat swords [because of their strength]), elements, tactics, classes, and so on. Such systems quickly defy diagramming as more options are available (Pokemon comes to mind, with
seventeen eighteen elements in the video games), but the underlying concept is still there.