A Fruitful Problem

There are three boxes. One has apples, one has oranges, and the other has apples and oranges. The boxes are labeled wrong, so that no label is correct. Sue opens just one box, and without looking in the box, takes out one piece of fruit. She looks at the fruit, and immediately labels all the boxes correctly.

Which box did she open, and how did she know?

The Answer: If Sue had reached into the box marked 'Apples and Oranges', and pulled out an Apple, then she would know that the box should be labled 'Apples'. Knowing that the other two boxes are also labeled incorrectly, Sue would label the box marked 'Oranges' with 'Apples and Oranges', and the box marked 'Apples' with 'Oranges'. Get it?