It's one of life's knotty problems -- You put on shoes, tie the laces up as firmly as possible, but soon after the laces come loose. Now scientists think they know why.
After a series of experiments using a slow-mo camera, mechanical engineers at UC-Berkeley found "shoelace knot failure" is triggered by a complex interaction of forces, and happens in a matter of seconds.
The foot striking the ground stretches and relaxes the knot, while the swinging leg acts like a hand pulling on the ends of the laces.
While some laces might be better than others to tie knots, all suffered from the same fundamental cause of knot failure, the study, which was published in the journal Proceedings of the Royal Society A, found.
Lead researcher Christopher Daily-Diamond 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."
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