It expands. You change your constraints on your system, you heat it up some more, then you take the heat source away, and you put it back in contact with the atmosphere. And you cool it a little bit, change the constraints, cool it a little bit more, and heat, and you've got a closed cycle engine.

让它与大气接触冷却,改变约束，使它继续冷却，然后又再次加热,这就是循环热机，我们以前也研究过，更复杂的机器。

麻省理工公开课 - 热力学与动力学课程节选