You must have heard that police uses lie detectors on criminals to detect the truth. John A. Larson, a medical student from California University, developed a machine in 1921, which can detect lies with a fair degree of accuracy. This machine is known as ‘polygraph’, or a ‘lie detector’. In 1972, the American inventor Allan Bell developed another improved model of a lie detector, which was able to detect slight trembling in the voice that often occur when a person tells a lie. Today, it is widely used in crime detection. How does this machine detect lies? When someone tells a lie, there occur certain physiological changes in his body. His heart begins to beat faster, leading to rapid perspiration. His blood pressure and breathing pattern change. His voice also starts trembling. The lie-detecting machine is designed to record all these changes. When a person undergoes a lie-detector test, various parts of the machine are attached to his body by wires. The interrogator asks the person all relevant questions and the physiological changes taking place while questioning are recorded by a pen recorder fitted in the instrument. Finally, these observations are analysed. However, the polygraph is not wholly dependable. Its accuracy is estimated to be about 80%. In fact, a lot depends on the skill of the examiner in this regard. Some people, who are truly unaware of the fact that they are lying, cannot be caught by the lie-detector. Moreover, the hardened criminals can control their emotions and render this machine ineffective. Because of these reasons, the results of a lie-detector test cannot be accepted as evidence in the court of law. However, it is being used by the police departments in almost all the countries.