0Vyger 8 years ago The device is called a conscious. Almost everyone is taught from childhood that it is wrong to lie, so like it or not your brain and thus body react to telling a lie. You know that you are doing something wrong and thus it makes you nervous and uneasy. That reaction is what is detected, although it can sometimes be difficult to tell if you are nervous about the process, something else, or if you are just lying. That is why it takes a trained person to interpret the results and even then they can get it wrong which is why a lie detector is not considered to be valid evidence in court.