Headscratchers / The Fly (1958)

• The protagonist and the fly switch heads, but retain their memories and (most of) their personalities. How?
  • Just because the exterior heads were changed doesn't mean the brains were (entirely, at least).
  • To be fair the scientist's mind was slowly deteriorating to that of a fly, which is one of the many reasons he was so desperate to turn back. Maybe the fly's mind did the same and ended up, as we see at the end of the movie, as pure human.