In this fun little trick, David Blaine ties his shoelaces just by shaking his leg. Not one of his greatest achievements, but the kids seem to be impressed!
This trick requires a little preparation but is well worth it. You will need a fake shoelace which should, partly covered by your trousers, create an impression of your shoelaces being untied. You should be able to quickly hide the fake shoelace from the eyes of the audience. You can achieve that by tying a thread to the centre of the shoelace. Just place the thread down your trouser leg. You then hold the other end of the thread in your hand and pull it when you want to hide the fake shoelace. The real shoelaces are tied the whole time, but hidden behind your trouser leg. You uncover them by shaking your leg.
So, when performing this trick, just start shaking your leg, pull up the fake shoelace and be sure to make the real shoelaces visible.
If you find this explanation hard to understand, here is a video that shows exactly how to do it: