Three boxes are all mislabeled — one draw tells you what's really in every one
Three boxes hold, respectively, two black marbles, two white marbles, and one of each — but a prankster has swapped every label, so all three boxes are now guaranteed to be wrong. You may reach into just one box, without looking, and pull out a single marble. From that one marble alone, how do you correctly relabel all three boxes?
Reveal the answer
Draw from the box labeled 'black and white' — since every label is wrong, that box must actually hold two of the same colour. Whichever marble you pull tells you that box's true contents, and from there the other two boxes' true labels are forced by elimination, since they also can't match their current (wrong) labels. Martin Gardner published the puzzle, sometimes called 'Scrambled Box Tops', in his Scientific American column.