Originally Posted by BikeWise1
Cables don't stretch, either, but housings compress, giving the illusion the cable got longer. Don't believe it? On a bike with well used housings, take a brand new derailleur cable and run it through the old housing and adjust the derailleur. Now go ride and note how it stays adjusted! The housings were already fully compressed. All major brands of cables are prestretched in a controlled fashion at the factory. No need in the shop. But we always compress the housings!
This is not quite right either. In fact what happens is that the cable wears its path into the liner inside the housing. On a new bike, the cable wears a groove into the metal that it runs over. Grabbing the brake levers hard after installing new cables take up the slack that may exist in the run from lever to brake, and permits the cable to "slot" into the old wear pattens.
I'm also interested in seeing a "cable stretcher". It must suspend from the ceiling so someone can dangle off the end of the cable... because I can't see a cable ever stretching otherwise. Go figure it -- test the amount of force required to move a derailleur or engage a brake caliper. Then go attach the end of a cable to something immoveable, and try to "stretch" it. Good luck. Now if that poster had said "a tool for taking up the slack in a cable system" then I might believe him.
Anyone who says a chain or cable used on a bicycle is stretched, is playing with themselves. The key for the OP was "I was told by a young kid at a bike shop".