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:

my solution depends on what is meant by "inside the cave".  I propose that you have the club memebers stand between the entrances to tunnels 1 and 2 and face towards the entrance to tunnel 4.  This is assuming you have the key to either 1 or 3, if that is not the case then have them face the opposite direction.  Now enter the tunnel for which you have the key then exit via door 4.  They will see you come out of tunnel 4.  Now they don't know which of 1 or 3 you have the key too and since they were watching tunnel 4 the whole time they know you did not enter it, yet you left through it, thus they must conclude that you were able to enter the room in order to exit out of door 4.

Posted by Daniel on 2008-12-27 03:13:52