In physics, you can apply the zeroth law of thermodynamics to compare the temperatures of multiple objects. Two objects are in thermal equilibrium if heat can pass between them but no heat is actually doing so. For example, if you and the swimming pool you’re in are at the same temperature, no heat is flowing from you to it or from it to you (although the possibility is there). You’re in thermal equilibrium. On the other hand, if you jump into the pool in winter, cracking through the ice covering, you won’t be in thermal equilibrium with the water. And you don’t want to be. (Don’t try this physics experiment at home!)

To check for thermal equilibrium (especially in cases of frozen swimming pools that you’re about to jump into), you should use a thermometer. You can check the temperature of the pool with the thermometer and then check your temperature. If the two temperatures agree — in other words, if you’re in thermal equilibrium with the thermometer, and the thermometer is in thermal equilibrium with the pool — you’re in thermal equilibrium with the pool.

The zeroth law of thermodynamics says that if two objects are in thermal equilibrium with a third, then they’re in thermal equilibrium with each other. Then you can say that each of these objects has a thermal property that they all share — this property is called temperature.

Among other jobs, the zeroth law sets up the idea of temperature as an indicator of thermal equilibrium. The two objects mentioned in the zeroth law are in equilibrium with a third, giving you what you need to set up a scale such as the Kelvin scale.