You have fifteen piles of coins, each pile containing twenty coins. The coins appear to be identical in…?

every respect; however, they are not. Fourteen of the piles contain coins that weigh 2 grams each, but one pile is counterfeit, and those coins weight 2.1 grams each. You have at your disposal a single pan scale, such as is used to weigh produce, and your problem is to determine which of the piles contains the counterfeit coins, using the scale once and only once. You may not add or take away coins once they have been placed on the scale.