I’m at logic camp.  Here’s a simple, fun puzzle that I learned:

You and your friends Andy and Brandy—two friends who happen to be pretty good at reasoning—are all wearing hats.  A third friend, Candy, has such a deft touch that she can place a feather in one’s hat without them knowing.  The hats are brimmed, so if there is a feather in it, the hat wearer would not be able to see.

Candy has only two feathers, and can do with them what she pleases.

You are standing single filed: You in front, Andy behind you, and Brandy behind her.  In such a configuration, Brandy can see whether or not Andy’s hat is feathered, and both Andy and Brandy could see a feather in your cap, were there to be one.  You can see the wall in front of you, which won’t help you solve the puzzle.

Candy—after maybe placing some feathers into hats, but maybe not—asks Andy and Brandy if they know whether their respective hats have feathers.  They both say “No”.  Candy repeats her question.  This time, Andy says “Yes, I know whether or not there is a feather on my head”.

Is there a feather in your hat?