One player is the leader or 'St. Valentine' and orders the other players to make motions by saying "St. Valentine says" and then an action. For example they might say "St. Valentine says thumbs up", "St. Valentine says hands on your head" or "St. Valentine says jump up and down." The leader also makes all of these motions. If he omits the "St. Valentine says" part, however, and just orders "Thumbs up" or "Jump up and down", the players must not make the motion. Anyone who does so is out of the game. The winner is the player who remains in the game the longest.
Step up version:
In another version, "St. Valentine" can stand about 5 feet in front of the group which is lined up in a straight line. Each time someone messes up, the others move forward a step. Eventually it will be whittled down to one person and they get to be the next St. Valentine.