We know that The Zeroth law of thermodynamics is-
If an object A and another object B are both in equilibrium with each other; then other object C which is in thermal equilibrium with object B will also be in equilibrium with object A and the temperature of object C is equal to the temperature of object A.
Now, when we measure our body temperature using a medical thermometer, We know 1st object is our "Human body" and 2nd object is " The thermometer" but what is the 3rd object that is equilibrium with the body and thermometer?
I heard people saying, if the first object is human body and 2nd object is the thermometer (which we use to measure the temperature for fever), so the third object must be the environment(surroundings). But I have issues with that. Why it has to be the environment. Thermometer bulb, Thermal substance(here:Mercury), and our body can be that 3rd system. only if we need to. But 2 systems can also be in thermal equilibrium with each other.