ELI5 Lie Detector Tests - how do polygraph tests work?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

How does a lie detector test work? If people don’t trust their accuracy, why do agencies like the FBI and CIA still use polygraph tests? How is a polygraph test actually conducted? Are there any proven methods or techniques to "beat" a polygraph test?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: amainterrogator, rhomboidus, slightydamp_clothes, jetscootr, happy_bridge, baliflipper,jarljam and antechronos

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