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How Does a Lie Detector Work?  Y­ou hear about  lie detectors  all the time in police investigations, and sometimes a person applying for a job will have to undergo a polygraph test (for example, certain government jobs with the FBI or CIA require polygraph tests). The goal of a lie detector is to see if the person is telling the truth or lying when answering certain questions. When a person takes a polygraph test, four to six  sensors  are attached to him. A polygraph is a machine in which the multiple ("poly") signals from the sensors are recorded on a single strip of moving paper ("graph"). The sensors usually record: Sometimes a polygraph will also record things like arm and leg movement. When the polygraph test starts, the questioner asks three or four simple questions to establish the norms for the person's signals. Then the real questions being tested by the polygraph are asked. Throughout questioning, all of the person's signals are recorded on the m