The principles of polygraph operation are explained to be the scientific measurement of the subject's breathing rate, blood pressure, and galvanic skin response. The basic assumption in using the polygraph to detect lying is noted to be that the subject's breathing, blood pressure, and skin condition change according to whether or not he/she is telling the truth. Polygraph techniques are explained while portraying a training session for potential polygraphers. The procedure for developing 'neutral' or 'control' questions is depicted as the means for measuring the subject's truth responses in comparison to any lying responses to 'relevant' questions (questions related to the reason for the test being administered). Polygraph reliability is defended by polygraphers; they claim unreliable tests are due to flaws in technique and interpretation of results rather than in the machine and the principles of its operation. A psychologist who challenges the polygraph's reliability argues that it only measures changes in biological processes, but it cannot determine the reasons for these changes. A demonstration shows that taping a tack to the big toe and pressing it into the flesh when responding to control questions reduces or eliminates the contrast between truthful and deceptive responses. The case of an innocent man being convicted of aggravated murder and given a life sentence based largely on the testimony of two polygraphers is used to demonstrate the unreliability of the polygraph and its possible consequences in criminal cases. The pros and cons of using mandatory polygraph testing in preemployment screening, particularly for positions that involve handling money or state secrets, are considered.