Here it is:
Given 12 coins, all of the same weight except one, and using only a two-pan balance, find the odd coin and determine whether it is heavier or lighter than the others, making only three weighings.
This puzzle has an interesting history. The Statistical Research Group was established in World War II to use statistical techniques in the war effort. Many of the people in this group went on to become really heavy hitters in statistics. One of them, Milton Friedman, won the Nobel prize in Economics, a branch of statistics. The story goes that some of them spent several weeks discussing this puzzle during lunch. Apparently most of them were unable to solve it, but you have to remember there was a war on. One of them eventually came up with an elegant solution to the general class of coin weighing problems, though. Bet you didn't know there was a general class of coin weighing problems.
http://www.jstor.org/stable/pdfplus/2287451.pdf
and here's an Excel spreadsheet to help you if you want to try to solve the problem:
You might encounter two slight complications in using this spreadsheet: First, if you have an older version of Microsoft Internet Explorer, you'll have to first save the file, then open it. This doesn't apply to Firefox, nor, I think, the newest version of Internet Explorer. They seem to have fixed the problem in the older versions of opening up Excel spreadsheets over the web, hard as that may be to believe. Second, it contains Visual Basic macros, so you'll have to enable them when you open the spreadsheet. This may involve change your security settings in older versions of Excel or enabling Visual Basic in the 2007 version.
Make sure you read the instructions.