This. They measure these body changes and some idiots decided that these correlate perfectly with lies, but really they can correlate with plenty of things. They start with a base measurement where they ask you simple and verifiable questions such as your name, address, etc.

Here are some legitimate reasons that can get you marked as a liar:

• Getting nervous because you are being investigated.
• An accusatory question gets you nervous
• Panic attack
• Physical discomfort, can be because of a long investigation.
• A question agitates you
• And more

A polygraph can be useful to help uncover the truth as it can help investigators possibly find subjects that disturb you and could relate to lies.

Calling a polygraph a lie detector is ignorant, malicious, stupid, or some combination of the above.