The following is a fun puzzle that LindsaySnider gave me and I changed the story for, because it didn't have enough ducks and evil overlords.
He will let you talk for an hour, then he will send each of you to different corners of his castle so that you may never communicate again. But he will let you all go once you have all seen a particular magical garden. But since he's evil, there are tricks.
He will randomly take people one at a time to the garden (multiple trips are possible and since he's evil, likely). In order to actually be let go, *YOU* (just your luck, right?) have to tell the evil overlord that you have all seen the garden the very first time that you see the garden and everyone else has seen the garden too.
The magical garden is truly magnificent, but you don't get to go into it, as you are a filthy foreigner. Just before the magical garden are two reflecting pools, each with a duck that is exactly the same.
The ducks move around in the pools but never leave them, if extracted they never re-enter the pool unless someone puts them back in. You can put a duck in a pool, or take a duck out of a pool, but you can not put two ducks in the same pool. The evil overlord guarantees that you can only tell if a duck is in a pool or not in a pool (i.e. you can't paint the duck or kill it or any of the stupid things that other stupid foreigners have tried before being summarily executed by the overlord). Furthermore, he tells you all that you *must* move one duck (and only one duck) each time one of you visits the magical garden.
How will you know when everyone in your group has seen the magical garden by only moving the ducks?