"Fire, just as the early humans did. My methods are a fair bit more focused by modern scientific understanding, of course..."

Yeah, except that they're not. The configuration of wood and glass and friction makes sense in Doc's mind, but it's all moving rather slowly. His hands twitch as he urges it onward---and, miraculously, there's a spark!

"There it is! Now to feed it...carefully, carefully..."

It's going to take a while to build, but he's fine doing it. As he works, he smiles up at Marty---with all that thick fluff of blanket, he looks like a marshmallow with a human head. Not that he'll ever mention this.