You have eight pennies that look the same, but one is counterfeit. The counterfeit penny is known to be heavier than the others. How can you tell which one is counterfeit by using a balance scale only twice? (The balance scale has two pans but no weights; you can place any combination of pennies in either pan and then observe whether the pennies in each pan weigh the same, or if one collection is heavier than the other.)

The original question did not include the information that the counterfeit penny is heavier, but without that information not even the MathNerds could do it. Can you?

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