Not a literal fire. [She's honestly not sure how microwaves work, precisely; even if she did, she suspects it would be fruitless to try and explain.]

And an oven...here, follow me. [Field trip out to the big kitchen! Evy walks over to the oven and opens the door, showing Athelstan the inside.] Instead of a--a cook fire, you can heat this up and shut food inside it. There's a range on top for pots and pans.