The Teenagers From Outer Space (TFOS) system is a lightweight rules system for comedic roleplaying, originally developed by Mike Pondsmith and Greg Costikyan for the first edition of Teenagers from Outer Space (1st, 2nd & 3rd Editions). The system has remained basically unchanged for all editions of TFOS, and was licensed by Dream Pod 9 for the Star Riders roleplaying game. TFOS is d6-based.
Character generation is mostly randomized. Players choose whether a character is human or alien and assign Knacks (skills), but Stats (ability scores), starting Knack points, and alien abilities are randomly rolled. Character abilities scores range from 1 to 6; starting Knack scores only range from 1 to 3, but may be improved by experience.
Task resolution in the system is accomplished by rolling 1d6, adding the relevant Stat (and relevant Knack, if any), then comparing it to a difficulty number set by the referee. (Opposed checks where two players roll against each other are also possible). Exceeding the difficulty number indicates the character succeeds at the task, but exceeding the difficulty number by "too much" (an amount determined by the gamemaster) creates comedic side-effects.
Like many comedy roleplaying systems, TFOS forbids killing player characters.