What is the distinction between pop and rock? In other words, what makes pop Pop?

The title is the question, but here's some context

My colleague is a metal head, and we were discussing different genres and what separates them, as I am notoriously bad at labelling metal sub-genres. We came to Power Metal, and he very quickly wrote it off as "just pop music with metal guitars" and... Whilst I would debate it and say there's more to it than that, I can't deny that it's certainly on the end of the metal spectrum that theatre kids would enjoy. But that then inspired the question of "what makes it "just pop"?" The main answer was that it was easy to listen to and had a pretty standard structure, the old verse-chorus-verse-chorus-bridge-chorus line. Except that most rock songs follow that same structure. So we went through pretty much everything we could think of, from instrumentation to vocal style, and every single thing you can fit into both pop and rock music.

So what do you guys think? Where is the distinction? What makes rock separate from pop?