The polygraph can't actually measure anything that can (with any reliability) indicate whether you're telling the truth or lying though. It's basically just a prop to try and stress the subject out and convince them that the examiner can tell what they're thinking. The examiner (really interrogator, calling it a "test" is too generous) will literally pick random things that they feel you're being evasive about and drill you on it, even if it has no relation to the truth!

What does admitting you had sex with a goat have to do with if the machine works or not? If I was not clear, they are looking to see if you can be blackmailed. They may know already you buggered a goat, or snorted a kg of the white stuff. They just want to know you will admit anything you have done as that is the key to make sure others cannot gain leverage over you.