You take the third one, and you go from one to the other, and you see whether heat flows, when you touch one object, the middle object, between those two objects.

第三个物体就可以作为温度计,将它与第一个物体接触，再与第二个物体接触，观察它们之间热量流动的方向。

麻省理工公开课 - 热力学与动力学课程节选