You're done with your iPod, so you carefully coil the headphone cord around the player and stuff it in your pocket. The next time you take it out, however, you find that you have to unravel the cord and undo a knot before you can go back to listening to your music. In fact, it's pretty amazing how easily knots can form by themselves-not only in headphone cords, but also in necklaces, coiled ropes, strings of holiday lights, hanks of yarn, or garden hoses.

Two physicists at the University of California in San Diego recently decided to do some experiments to try to find out why knots form so easily in coiled strings. They looked at what happens when a long string is coiled into a box and the box is then tumbled.

The researchers, Dorian Raymer and Douglas E. Smith, found that complex knots can form within seconds-if the string is long and flexible enough. So, for a given stiffness, a string has to be a certain length before a knot will form. Moreover, the longer and more flexible the string, the better a chance it has of becoming knotted, especially when the string is tumbled or shaken for a long time.

The researchers did the experiment over and over again: 3,415 times in all. Knots formed in the strings about one-third of the time. The biggest surprise came when the physicists used mathematics to identify the types of knots that formed in the strings. Mathematicians called knot theorists have described many different kinds of knots, according to features such as crossings (the number of times a string crosses over itself). But to the mathematicians, these knots were just an abstract idea. The physicists found that their real-life model produced 120 different knots, representing many that had only been theorized before. In fact, their strings formed all of the possible kinds of knots with up to 7 crossings-and 7 crossings is quite a tangle!

Raymer and Smith think that knotting tends to start from one end of a string. As a string tumbles, this free end braids itself with strands lying next to it, weaving over and under adjacent segments to create a knot. The more braiding, the more complex the knot.

So, if your headphone cord is long, thin, and flexible (if?!), the chances of it becoming knotted are annoyingly high. Sigh.

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