Why do cords and headphones tangle so fast?

The answer

In an experiment, scientists tumbled string in a box for ten seconds, to copy a bag being jostled. String shorter than about forty six centimetres never knotted at all. As the string got longer, the chance of a knot shot up, and with very long string about half of the tries ended in a knot. A phone charger or a headphone cord is well over that short limit, so it can tangle within seconds. The knots were not simple ones, either: complex knots formed within seconds, with nobody tying anything. This case came from a viewer request from Sam, whose headphones came out of an empty bag in a huge knot after only minutes.