Zeroth Law of Termodynamics

The Zeroth law of thermodynamics is, in my opinion the easiest law. It states that if two systems are in thermal equilibrium are both in thermal equilibrium with a third one then they are in thermal equilibrium too. Thermal equilibrium means that two systems wont have a flow of heat energy into one another if they are connected with a hollow tube or anything like that. So, basically they have the same heat and temperature. Now, the law says that if system A has the same temperature as system C and system B has thermal equilibrium with C then A and B also will be the same. It is very easy as it just says that if a = c and b = c then a = b. Surprisinglly though, it has many uses. The most important one is that we can compare temperatures of objects without needing to have them together. So, if we put a thermometer in A and wait until It gets in thermal equilibrium (lets say 50 degrees) then if we put a thermometer in B and it also goes up to 50 degrees then both A and B have equal temperature. You may think that it is obvious, but before we never knew if this was definitely true for any object and this law proves it.