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Suppose if there are three boxes, one contains only apples, one contains only oranges, and one contains both apples and oranges. The boxes have been incorrectly labeled such that no label identifies the actual contents of the box it labels. Opening just one box, and without looking in the box, you take out one piece of fruit. By looking at the fruit, Tell me how can you immediately label all of the boxes correctly?
Answer:
Open the box that is labeled "Apples and Oranges".
You know that since none of the labels are correct, the box must either contain only apples, or only oranges.
Suppose that you remove an apple from that box. Therefore, that box must be the "Apples Only" box.
One of the two remaining boxes must be the "Oranges Only" box. However, one is labeled "Apples Only", and the other is labeled "Oranges Only". Therefore, the one labeled "Apples Only" is the box that contains only oranges, and the box labeled "Oranges Only" is the box that contains both kinds of fruit.
You know that since none of the labels are correct, the box must either contain only apples, or only oranges.
Suppose that you remove an apple from that box. Therefore, that box must be the "Apples Only" box.
One of the two remaining boxes must be the "Oranges Only" box. However, one is labeled "Apples Only", and the other is labeled "Oranges Only". Therefore, the one labeled "Apples Only" is the box that contains only oranges, and the box labeled "Oranges Only" is the box that contains both kinds of fruit.