# How to know which head belongs to which tail in a bunch of wires? [duplicate]

I saw a puzzle in a contest in Iran held by an association called "Rahneshan" and the original text of the contest is uploaded HERE and of course both website and text are in Persian.

Between "City A" and "City B", there is a big cable which contains 100 separate wires inside it. Unfortunately, the wires are knotted together and we can't tell which head of a wire in one side (City A), belongs to which tail on the other side (City B).

The only tools that we have are:

• A power outlet
• A test light (which can be used to determine if electricity is flowing through a wire)

We can make a round-trip from City A to City B and back once.

How can we determine which endpoints belong to which wires?

• No I didn't create it myself. There's a kind of contest and I was asked. Commented Oct 18, 2020 at 21:19
• No it's not active. I was only curious about it and I want to see how each individual may solve the problem, if possible. I myself have no idea. Commented Oct 18, 2020 at 21:24
• To whoever closed it, this is not a duplicate. The alleged duplicate question asks "What is the fewest number of trips from one end of the cable to the other required to identify ...each individual wire." This question stipulates only ONE trip can be made. Commented Oct 18, 2020 at 23:12
• @Earlien Yes I think so, too. Thanks for your attention. But the procedure is completely useful in this puzzle, too. Commented Oct 18, 2020 at 23:15
• The answers to the linked question are answers to this question as well. They lay out a strategy, requiring one round-trip, that can solve the problem. Since the answers are the same the focus on a different part of the solution does not matter. Commented Oct 19, 2020 at 3:09