How do shoelaces come untied?

Have you ever been walking around when you notice that your shoelaces have come untied, even though you just tied them? What’s up with that? We asked engineer Christine Gregg to help us understand.  Send us your questions for Moment of Um at BrainsOn.org/contact, and we’ll do all the footwork to find the answer. See omnystudio.com/listener for privacy information.