"If two bodies or systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other."

Do you know Why is it called the zeroth law of thermodynamics?
Actually the other thermodynamic laws (the first, second, and third) had already been named, by the time this one was formulated. However, scientists thought that it was so important that it should go before all the others, and hence it was termed the zeroth law.
Explanation

Consider three systems or bodies A,B,C repectively all at different temperatures. Now if A is brought in contact with C, after some time the bodies A and C would be in te state of thermal equilibrium (at same temperature level.)

Similarly if B and C are brought in contact of each other, after some time they will be in thermal equilibrium. Thus , if A=C and B=C we can conclude that A=B ie. A and B too will be in thermal equilibrium.Hence the law “If two bodies or systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

Examples of uses Zeroth law :
1. The most popular example is the thermometer. We can observe the zeroth law in action by taking a very common thermometer having mercury in a tube. As the temperature is increased this mercury expands since the area of the tube is constant. Due to this expansion, the height is increased. Now, the increase in the height of the mercury label shows the changes in temperature and basically helps us to measure it.
2. Similarly, another example of the zeroth law of thermodynamics is when you have two glasses of water. One glass will have hot water and the other will contain cold water. Now if we leave them in the table for a few hours they will attain thermal equilibrium with the temperature of the room.

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