How does a lie detector work?

They're not really very accurate and you can get around them. It's called glavanic skin response. A lie detector works by measuring changes in skin conductance on the basis of sweating. It uses the fact that when you lie, your skin usually goes up in its conductivity because you sweat, and you sweat because you're nervous. This is also linked to blushing. There's another group of researchers who in the past few years have been looking at another way to tell if you're lying, which is studying closely the blood flow across the face. Although this is probably still undergoing tests, they found that when people tell a lie, the blood flow around the eyes specifically changes and increases blood flow. Even if your eyes aren't sensitive enough to pick it up, a clever camera can. So you can look at the heat or thermal changes in someone's face and tell whether they're trying to hide something. This would be useful, say, at the airport. If someone's checking in and says no when asked if there's anything in their bag, this may be a way to flush out the liars without having to do anything invasive.