# Phones Puzzle

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Object: Pick up the correct phone.

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One of the phones on the left is red.

Your task is to click the phone on the right that is connected to it.

Here is how you find the correct phone on the right:

Starting with the red phone on the left, go right along the horizontal bar until the next intersection with a vertical rung.

Follow this rung up or down to the next horizontal bar, then go to the right again to the next intersection, and so on.

If you pick up the wrong phone you lose.

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Contributed by: Karl Scherer (April 2014)
Open content licensed under CC BY-NC-SA

## Details

A Surprising Fact

It is not obvious, but no matter how randomized the board setup is, and how often the connections from the left to the right intersect (when you compare the connections between pairs of phones), each phone on the left is always paired to one single phone on the right and vice versa. Can you prove that this one-to-one relationship always holds?

When such connection problems are published in a newspaper, usually the connections consist of very curvy cables that have crossovers, but are always meant to be separate (in 3D) and do not use common parts of the connecting wires.

In such cases the one-to-one relationship is obvious and trivial.

This Demonstration shows a very different approach to pairwise connections, namely one where the connecting wires use an unlimited number of common parts. Here the one-to-one relationship is not obvious at all.

Controls

You have the choice of having 80 vertical bars or 120 vertical bars in your randomized setup.

To pick another phone on the left border, simply click one that is not red.

"reset"

Resets the game to the starting position.

"scramble"

Randomizes the setup.

Background Graphic

Fractal by the author; created with the Fractint freeware.

History

This game was originally published as the Zillions game "Phone 2" by the same author.

Sometimes this type of puzzle is presented tilted by 90 degrees, then a set of ladders is used that have to be climbed from the bottom to the top.

## Permanent Citation

Karl Scherer "Phones Puzzle"
http://demonstrations.wolfram.com/PhonesPuzzle/
Wolfram Demonstrations Project
Published: April 28 2014

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