Question: There are three boxes labeled "Apples", "Oranges", and "Apples and oranges". Each box has the wrong label on it. How could you correctly label all three of the boxes by taking only one piece of fruit fruit from one of the boxes?
Answer: A piece of fruit would be taken from the "Apples and oranges" box. Since this is the wrong label you know whatever fruit you draw from this box is all the same and you can give that box the proper label. Since all of the boxes started out wrong you know that the last box with its original label is wrong and can be labeled "Apples and oranges". From here you can just put the final label on the final box.