Separate the balls into 3 groups of 3. Place 3 balls on the left side of the scale and 3 balls on the right. If they balance out, you know that the heavier ball is not one of those 6. If one side is heavier, you know that the heavier ball is one of those 3.

Now for your second use of the scale, place one of the remaining 3 balls on the left and one on the right. If they are equal, then it is the ball that was left out that is heavier. If either side is heavier, then that is the heavier ball.

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