system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what's Happening when I'm by myself but it feels/sounds like someone just entered the room?
gpt
Its a sensory illusion. misinterpretation of external events and internal thoughts. Your brain doesn't take chances with your safety, it encourages you to check just to be sure. Since the only thing that would likely make you react in that way is another person, you momentarily think there may be anohter person who just came in. 

Basically, your brain gets confused and responds to its own confused responses, in an attempt to keep you from being killed.