system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does my body know if I'm 'being watched'
gpt
It doesn't.

There have been lots of studies on this, and - in all of them - the results show that people actually *don't* know they're being watched at all. They only get it right about 50% of the time: exactly what random chance predicts.

The "feeling" you get, though, can partially be explained in an evolutionary sense. We humans only have eyes on the *front* of our head, which means we always have a 'weak spot' behind us. It makes sense for us to be nervous about that area, since predators could be lurking there. Even if it turns out to be a false-alarm, it's still an evolutionary advantage to double-check, so it would be considered beneficial.

Interestingly, the act of 'checking behind you', often *makes* people look at you (because they've detected an odd head/body movement). When you make eye-contact, you then interpret this as meaning "Aha! You *were* watching me!" and it becomes a form of confirmation bias.