Answer:
Zeroth Law of Thermodynamics:
"If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other."
Application: Thermometer
Explanation: Suppose that we have two objects a chair and a table both are at the same temperature but we don't know if they are at the same temperature. Now we measure the temperature of the chair and the temperature of the table separately. We find that the temperature are the same.
According to Zeroth law of thermodynamics, the chair and table must be in thermal equilibrium with each other.
[As the chair is in thermal equilibrium with the thermometer and the table is also in thermal equilibrium with the thermometer]