[Jimbee (OP)]

I've noticed, and I know I'm not the only one, that a film sometimes forms on hot liquids that have been standing around for a while. I know they say that happens often to soup. But I knew if I'd say that someone would ask "which soups?" So I will just say milk. Usually whole milk I've noticed. And sometimes you can just stir it back in and sometimes you can't.

What is that film? And why does it just seem to form on hot liquids like hot milk?