So, for example, if the teacher asks "why does fire burn" and I answer "because it's hot", it feels like a real explanation

Looks like a real explanation to me: you can use it to predict that non-hot things do not cause burns, and that non-fire hot things (for instance boiling water) do cause burns.

It's a sufficient theory to avoid burns by moving in your hand slowly until you feel some heat, rather than directly grasping things that might be hot.

If you observe that friction heats things up and you are very persistent you will be rewarded with a way of making fire. The model is getting more sophisticated ("fire is hot; hot things burn; rubbing things heats them; hot things catch on fire") and perhaps at this point starts deserving the "mental model" tag.

You'd start needing moving parts when asked to predict e.g. what boiling water will do to a piece of paper. "Burn" is then wrong, for non-trivial reasons.