If we build an engine for a car or anything else, it almost always is going to have some key element that's operating in a cycle, otherwise it won't keep going, right.

如果我们为一辆汽车，或其他什么东西制造一台引擎,它总要有一些关键，要素来形成循环。

麻省理工公开课 - 热力学与动力学课程节选