I have in front of me three boxes. One contains all red balls. One contains all blue balls. The third contains a mixture of red and blue balls. The three boxes are labeled as to their contents, and the three labels correspond to possible contents of a box, but you know that the labels are all misplaced.
You may draw one ball (at random) out of a box of your chosing. After that, you must rearrange the labels so that they correctly reflect the contents of the boxes. How do you do this?