Puzzle: There are three boxes. One is labeled apples, another is labeled oranges. The last one is labeled apples and oranges. You know that each one is labeled incorrectly. You may ask me to pick one fruit from one box which you choose. How can you label the box correctly?
Solution: We know that all boxes are labelled wrong. So apple + orange box either contains apples or oranges only.
Pick from that box.
If it is an apple ? Orange box contains both and apple box oranges.
If it is an orange ? Apple box contains both and oranges box contain apples.