You are in room A, where you can see the ends of 15 wires, which are labelled 1,2,.. ,15. In room B, you have the other ends of the wires and those ends are labelled A,B,...,N,0. You have with you a battery, a bulb and a lot of small wires. By going to room B, and then back to room A, can you tell which letter corresponds to which number? Room B is on a different floor, which can only be accessed using a lift.
What about the general case of n wires? (n > 2).