There are 4 doors to the secret Truth Club room, each one at the end of a tunnel leading there. Each of the members has a keycard to a different door. All the doors are unlocked from the inside, but to enter from the outside, one needs to swipe the door's keycard to open the door for a few seconds.

You have found a copy of one of these cards, and want to prove beyond a reasonable doubt to the club members that you can unlock one of the doors, however, you don't want to expose to anyone whose door it is. What can you do to show this? (preferably with no additional materials, and without anybody else inside the cave/tunnels)

(Assume the club members are honorable enough to follow their part of the plan you propose.)

Here is a map of the tunnels:

(In reply to possible solution by Daniel)

May I refer you to
The Great Tunnel Switch?

Consider that as background.  "Inside the cave" means literally that, the cave being the space enclosed within the brown "earth"!

Your suppositions are akin to what might/could have been posted at the link.  I, in my comments, have not gone down that road.

I think that if you can find a solution to that one this will be addressed easier.

That said, I know a solution exists for that at the link, but I seemingly don't have that source, and it would be hardly fair to Gamer  if and when  I could resurrect it to suggest it here.

Posted by brianjn on 2008-12-27 03:56:57