You have just received nine rare coins. You get an anonymous tip that one coin is counterfeit. The real coins weigh 10 grams each. The counterfeit coin looks just like the real ones but weighs 9 grams. The only scale you have is an old two-pan balance. What is the least number of weighings you can make and still be sure you know which coin is counterfeit?
(Source: Mathematics Teaching in the Middle School, Nov-Dec 1995)