Lie detectors cannot actually identify lies; all they can do is detect certain activities of a person’s body that are usually associated with lying: abnormal perspiration and flushing of the face, rapid breathing, increased heartbeat, and excessive swallowing.
Because this method does not actually guarantee the truth, the results are not generally accepted as evidence in court.
Although each of the body changes, alone, does not necessarily indicate a lie, when put together, they can provide an indication of one.
When a person is given a lie detector test, various parts of the machine are attached to his body, and a written record is made of his answers. Usually, three or four tests are given to be sure that the answers were not accidental.
Yet, amazingly enough, a lie detector can be fooled! There are some people who are truly unaware that they are lying. These people cannot be caught by a lie detector.