You have 12 marbles. One is either heavier or lighter than the others, but you don't know whether it's heavier or lighter. The only thing you can weigh them with to find out which is the marble with a different weight is a balance scale. So, how can you discover which marble has a different weight 100% of the time while weighing only 3 times? Hint: Weighing 6 against 6 doesn't do anything in this problem.