Engineering researchers say they have solved the riddle of why shoelaces come undone.
After a detailed study, they discovered that two forces act like invisible fingers, first loosening the knot and then tugging until laces are left trailing on the ground.
They used a slow motion camera filming a person running on a treadmill to figure out how the failure of the knot occurs in seconds after it is triggered by an interaction of complex forces.
It was found that the running foot exerts a force seven times greater when landing on the ground than the one exerted while standing, forcing the knot tying the laces to stretch and relax, loosening it slightly.
At the same time, as the knot loosens, the swinging of the laces that occurs as the leg moves forwards and backwards causes an inertial force to be applied on the free ends of the laces, pulling the already-loosened knot apart.
The researchers say the finding could have knock-on uses when applied to other intertwined structures, like DNA.
Co-author and graduate student Christine Gregg added: "To untie my knots, I pull on the free end of a bow tie and it comes undone.
"The shoelace knot comes untied due to the same sort of motion.
"The forces that cause this are not from a person pulling on the free end but from the inertial forces of the leg swinging back and forth while the knot is loosened from the shoe repeatedly striking the ground."
Lead researcher Christopher Daily-Diamond, from the University of California at Berkeley, said: "When you talk about knotted structures, if you can start to understand the shoelace, then you can apply it to other things, like DNA or microstructures, that fail under dynamic forces.
"This is the first step toward understanding why certain knots are better than others, which no one has really done."
Adding weights to the loose ends of a swinging laces showed that they untied themselves more often, as the inertial forces on their ends were greater.
The study also found that, while some laces might be better than others for tying knots, they all suffered from the same fundamental cause of knot failure.