And this occurs under constant temperature. So let's see what the layout of a heat engine is. A heat engine is composed of a long cylindrical tube that contains molecules inside this area and that contains a movable piston controlled by an outside force, maybe your hand that's moving it up or down. It also contains a hot body connected to the bottom. This hot body is important in conduction because remember, conduction requires for physical contact between two systems. And conduction allows heat transfer or energy transfer from a hot object to a cold object.