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The Zeroth Law of Thermodynamics states that if two thermodynamic systems are in thermal equilibrium with each other, and also separately in thermal equilibrium with a third system, then the three systems are in thermal equilibrium with each other. The zeroth law is used to define temperature and to establish the concept of thermal equilibrium. It allows us to measure temperature and to compare the temperatures of different systems.
For example, if you place a thermometer in a glass of water, the thermometer will eventually reach the same temperature as the water. This is because the water and the thermometer are in thermal equilibrium with each other. If you then place the same thermometer in another glass of water at a different temperature, it will eventually reach thermal equilibrium with the second glass of water. By observing the temperature at which the thermometer stabilizes in each case, you can compare the temperatures of the two glasses of water.