A Delhi court on Monday permitted the National Investigation Agency to conduct a lie detector test on Mohammad Naveed Yakub, a terrorist who was caught after the August 5 attack on a Border Security Force convoy in Udhampur, Jammu and Kashmir. The test, requested for by NIA after it said Naveed's statement contained lots of inconsistencies, was set to take place in court on Tuesday.

But does that mean that any information emerging from the polygraph test will actually be accurate? Until now, according to reports leaked to the media, Naveed has continuously changed his statements. After being captured, he has taken three different names. First he said his name is Kasim Khan, he then changed it to Usman and then again to Muhammad Naveed.

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As he gets set to take the polygraph test, it’s important to know how lie detector machines actually work. There are many sensors attached to the machine, which detects changes in the body such as heart rates, blood pressure and breathing patterns. Once the changes are detected, needles attached to the machine write reactions just like the cardiogram shows heart rate with many crest and troughs.

If you have a small reaction to the question, it’s a smooth line, while bigger jumps show bigger reactions. After the questioning, the interrogator starts looking at those target questions, which tends to show big signs of activity. That indicates that the person being interviewed has had a stress response to being asked that question, which could indicate a lie.

This approach assumes the standard response to lying or being interrogated. Unfortunately this simply isn't how the human body works. As this video by HowStuffWorks explains, even a normal person in stress could produce some signs of activity which according to the machine could be a lie, but in reality it actually isn’t. This is partly why lie-detector tests can only be used to help authorities in an investigation, and cannot be used to secure a conviction in Indian courts.