Marble

Games of involving marbles have contributed several phrases American use for situations that have nothing to do with game play. "Playing for keeps" in marbles means that shooters who knock their opponent's marbles outside the circle takes those marbles home. Playing for keeps also means undertaking an act or process of grave importance. Knuckling down refers to the manner in which a player shoots at target marbles, with the knuckles of the shooting hand firmly on the ground. But knuckling down also refers to applying oneself earnestly to a task. A player who has "lost his marbles" is out of the game. The phrase also suggests one lacking in common sense. It is interesting that a child's game has managed to invade important matters of the adult world.