I had three containers each with two balls. The one labelled WW contains two white balls, the one labelled BB contains two black balls and one labelled BW contains one white and one black ball.
Now the labels are scrambled so that the boxes are all labelled incorrectly.
If you are allowed to remove one ball at a time from boxes of your choosing without seeing what else is in the boxes, what is the minimum number of balls that must be examined to determine what each container contains?
(puzzle due to Martin Gardner)