Mark Frauenfelder found this puzzle in the book My Best Mathematical and Logic Puzzles.
Imagine that you have three boxes, one containing two black marbles, one containing two white marbles, and the third, one black marble and one white marble. The boxes were labeled for their contents – BB, BW, WW – but someone switched the labels so that every box is now incorrectly labeled. You are allowed to take one marble at a time out of any box, without looking inside, and by this process of sampling you are to determine the contents of all three boxes. What is the smallest number of drawings needed to do this?
Don't look in the comments if you are still working on it, because I believe someone will post the correct answer soon. -via Boing Boing