Question: How do lie detectors detect lies??

Ciarán O'Brien answered on 20 Nov 2014:

Lie detectors work* by measuring things like heart rate, the amount of sweat you’re producing, and your body temperature, involuntary muscle movements, things like that. All of these things can change when you’re lying. More recently lie detectors have measured your pupils (they dilate a little bit when you’re tense or concentrating, which can happen when you lie). When you’re hooked up to the detector, someone asks a series of boring or irrelevant questions that they already know the answers to (what’s your birthday? Is your name John? Things like that), to establish what your heart rate and muscle movements are like when you’re telling the truth. From there they can see any spikes in activity when you answer a question, which is a sign you may be lying about it.

Lie detectors aren’t very reliable though. All of the above change when you’re scared, and being questioned by police can be scary even when you’re innocent! It’s also possible to control some things like sweating and heart rate by keeping calm (or just not caring about being hooked up to a lie detector), and lots of people have been able to fool lie detectors like that.