adapted from an earlier Futility Closet posting:
You are given 9 coins, numbered 1 through 9, 8 of which weigh exactly the same, and 1 which weighs slightly less than the others. Given an accurate balance, how can you now identify the lighter coin in only two weighings?
...for the solution, see this posting at Futility Closet where it is noted that, "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.' ”):