Counterfeit coin puzzle

Futility closet has a good brain teaser:

You have nine coins and a balance scale. One of the coins is lighter than the others. Is it possible to identify it in only two weighings?

J.E. Littlewood observes that a similar puzzle wasted 10,000 scientist-hours of work during World War II. "There was a proposal to drop it over Germany."

Counterfeit coin puzzle