I love watching infomercials, but always wonder how much the sellers are exaggerating. Take this infomercial for the "Power Balance" bracelet, which is claimed to increase balance and coordination:
Now, go to this link which shows you how it actually works:
Make sure to watch the whole thing, because they give away the "trick" near the middle. It is useful to go back afterward and watch the first one, now that you know the trick.
The real question, then, is: what should you do if you know a friend is considering buying this, or worse, has already bought it? When I showed these videos in my class, I was told that the football team had purchased them already. When some of my students presented them with the evidence, their response was that they didn't care whether it worked or not.