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At the beginning of thermodynamics studies, it is commonly stated that a reversible process is one in which the system can be returned to its initial state without any change in the universe. Subsequently, it is claimed that an example of a reversible process is the heat exchanged between two bodies with a very small temperature dierence. However, clear links between these two statements are lacking. This article aims to bridge the gap between these two assertions by providing a clear and coherent link. The demonstration of the link is based on a simple yet powerful hypothesis: two identical bodies brought into contact will reach an equilibrium temperature that is the average of the initial temperatures (we will assume work is always negligible). A signicant thermodynamic result will be achieved with a straightforward (though somewhat labori- ous) mathematical approach. The article presents all the necessary cal- culations, allowing the reader to focus on understanding the underlying reasoning.