If you're like most people, lying makes your heart race. It makes you pant. It drives up your blood pressure and makes you drip sweat. A polygraph machine detects lies by looking for signs of these physiological changes.
There has been a lot of controversy about whether lie detectors work. Some experts claimed that a high proportion of persons who “failed” the polygraph subsequently confessed to crimes. On the other hand, the test generates a lot of false positives.