Andy, Beth, and Carl are excellent logicians.
I show them that I have a bag containing five hats -- three green and 2 purple. (I started to say red and blue, but wanted to stay apolitical :-) )
I line Andy, Beth, and Carl up in single file, and reach into the bag and put a hat on each of them. None of them can see the color of their own hat, and can only see to colors of those in front of them in line, if any. I.e., Andy can see no hats, Beth can see only Andy's, and Carl can see Beth's and Andy's.
I ask if any of them knows what color hat they have, and they all say "no"
I ask again and again they all say "no"
I ask a third time and someone correctly identifies the hat he or she is wearing.
Who answered, and with what color hat?