Columbo's minimal information problem.

As told to Lt. Columbo by Theodore Bikel.

You have bags of gold coins, any number that you wish. Each bag has the same number coins, however many you wish. One bag contains only counterfeit coins. The counterfeit coins either weigh less than the gold coins or more, whichever you wish. You have only a scale to use to find out which bag contains the counterfeit coins. The scale is operated by inserting a penny, and you have just one penny. How do you find the counterfeit coins with only a single weighing?