A normal person cannot bite their own finger in half because the body has things in place to prevent you from overly injuring yourself like that. You would stop yourself short and just cause pain, maybe bleeding, but no long term damage.
Now someone else’s finger is a different story. Also people with certain medical disorders can ignore the feelings stopping them from biting their own finger off.
I've always wondered if this is actually true, or if it just gets spread around as true because no sane person would actually test it with all their might lol
Or maybe it's true in the sense that "a normal person can't stab themselves in the face with a knife because the body has things in place to prevent that"