Right, answer time.

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Split the balls into 3 piles - two of 3 and one of 2.

Take the two piles of 3 and weigh them against each other. If one side goes down, then take two of those three balls and weigh them against each other - whichever side goes down is the counterfeit ball. Should neither go down then the third ball (the one you haven't weighed) is the counterfeit ball.

Should neither side go down when you put the piles of three on then the pile of two countains the counterfeit ball, in which case you just weigh the two against each other and whichever goes down is the heaviest and therefore the counterfeit ball.