The polygraph machine records changes in heart rate, breathing, blood pressure, and the electrical activity of the skin (galvanic skin response, or GSR). In the first part of the polygraph test, you are electronically connected to the machine and asked a few neutral questions ("What is your name?" "Where do you live?"). Your physical reactions serve as the standard (baseline) for evaluating what comes next. Then you are asked a few critical questions among the neutral ones ("When did you rob the bank?"). The assumption is that if you are guilty, your body will reveal the truth, even if you try to deny it. Your heart rate, respiration, and GSR will change abruptly as you respond to the incriminating questions..

That is the theory: but psychologists have found that lie detectors are simply not reliable. Since most physical changes are the same across all emotions, machines cannot tell whether you are feeling guilty, angry, nervous, thrilled, or revved up from an exciting day. Innocent people may be tense and nervous about the whole procedure. They may react physiologically to a certain word ("bank") not because they robbed it, but because they recently bounced a check. In either case the machine will record a "lie." The reverse mistake is also common. Some practiced liars can lie without flinching, and others learn to beat the machine by tensing muscles or thinking about an exciting experience during neutral questions.