system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people think there was a second gunman on the grassy knoll?

This conspiracy theory must have originated from somewhere. Is there any validity to it or has it been debunked? Thanks.
gpt
Very simply put: The film depicting the bullet striking the President in the head appears to show him being shot at an angle that does not marry up with the Book Depository, which is where Lee Harvey Oswald was when he fired his shot(s). 

It's theorised that had the round been fired (truly) from the Book Depository that the President would have suffered different wounds.