We've always wondered. They are securely tied one moment, and loose the next. What happened? Well, an international team of scientists at UC Berkeley and Aston University in the UK are on the case. And no MR, it isn't Gremlins. http://www.laboratoryequipment.com/...heory-science-shows-why-shoelaces-come-untied They enlisted volunteers on treadmills and robots that stomped and swung mechanical legs. Neither stomping or swinging by themselves caused the knots to come untied. But a combination of stomping and swinging did. Apparently stomping causes the knot to become tied less tightly so that slippage becomes possible in it. The swinging of the leg imparts lateral forces that pull on the laces. After enough of that a catastrophic failure ensues and the knot unravels very rapidly.