There are 9 balls that are identical. Only one ball is heavier than the rest. You also have a two-armed scale with which to measure the weight of the balls. How can you find the heavier ball by using the scale only twice?
Answer
Split the balls into 3 groups with 3 balls each. Pick 2 groups and use scale to determine which group contains the heavy ball.
Once the group is determined, pick 2 balls and use the scale. If they are equal, the other ball is the heavy one.