There is some sort of fundamental physical law that prevents cables from ever being just exactly the length you need them to be. Cables are always, always, just barely too short, or much much too long. Obviously, this annoys scientists just as much as it does the rest of us, so they've come up with a solution: stretchable cables full of liquid metal.
Making electronics that stretch is totally different (and much harder) than making electronics that bend and flex. With bending and flexing, the dimensions of the electronic components (like wires) don't change that much, but stretching is all about finding materials that can drastically change length or width without ruining the properties that make electronics functional in the first place.
Researchers from North Carolina State University have created a cable that can be stretched up to eight times its resting length by combining two separate materials: an elastic polymer outer jacket, and a gallium-indium alloy liquid metal core. You probably remember gallium from our epic geek gift guide: it's a metal that will melt in your hand, and alloying it with indium makes it even more flexible. You can't make wiring out of this stuff by itself without making a huge and potentially toxic mess, but jacketing it in elastic keeps it contained while it stretches. NCSU's insight was to stop trying to combine the metal with an elastic directly (which imposes some rather severe performance compromises), and just use the two materials in concert instead.
These new wires are fairly cheap and easy to manufacture, but NCSU still has to figure out some way of keeping the liquid metal from spilling out if the wires ever do actually break. Meantime, put all your doubts about how awesome this tech is to rest by checking out a super stretchy headphone cable in the video below.