They keep coming undone.
Mathematicians tell us that there are 43,200 ways of feeding a lace through the six pairs of eyelets on an average shoe.
There are also dozens of ways of making a knot that won't come loose after five minutes.
Probably the best is the double slip knot, which is almost impossible to explain without a series of diagrams.
Basically, it involves making two loops and then wrapping them around each other.
If you want to see how it's done, see www.fieggen.com/ shoelace/secureknot.htm for a quick lesson.