You have ten identical bags of identical coins – except that one bag contains counterfeit coins that weigh one gram less than real coins. The difference isn't enough for you to determine which bag has the bad coins by feel; you need to weigh them on your highly accurate scale.

You know the weight of a good coin (20 grams), and your scale can easily differentiate weighs down to a fraction of a gram.

If you are only allowed one weighing, how do you determine which bag has the counterfeits?