A fireless cooker is the insulated container in which hot, partially-cooked food is placed to continue cooking without further fuel — the haybox, the retained-heat cooker. The pot is brought to the boil, then nestled into the insulated box, where trapped heat finishes the job slowly: stews, beans, and porridge cook through over hours with no additional energy. The method is ancient — hayboxes warmed medieval kitchens — and radically efficient, using a fraction of the fuel of continuous simmering. It suits the patient dishes: the longer the gentle cooking, the better the result. Modern versions use vacuum insulation and sleek design, but the physics is unchanged — heat kept is heat used. The fireless cooker is slow food in its most literal form: cooking by insulation, finishing what the flame started.