If I'm going to do this puzzle thing, maybe I should start with the classics. You know, the puzzles that everyone has seen before, but maybe not everyone remembers how to solve. And then I can explain the solution. Heck, I can run off three or four of those this week, and maybe I'll be inspired to come up with better material.
So here goes: A coin collector has a case containing eight Spanish Doubloons, one of which is counterfeit. The coins look and feel identical, but the replica is slightly lighter than the other seven coins. The collector challenges you to find the fake coin, using only an old-fashioned balancing scale (the kind with two plates: you put something on one plate, you put something on the other, it tells you which is heavier), in as few weighings as possible. What is the minimum number of weighings needed to determine the fake?