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When you play for keeps, you don't just win marbles, you take the other person's marbles and laugh in their face while you do it.
I played for keeps and took all his marbles. He cried. I laughed. He still owes me a snack.
That kid said he was going to beat me. I played for keeps and now he's out of marbles and out of luck.
My brother tried to cheat me. I played for keeps and now he's stuck with his old, broken marbles.