As an autonomous discipline, thermodynamics has interesting limitations. First, it cannot understand its own basic observations: Why can the temperature be defined independently of any system? Why is there an absolute zero? What is entropy? What is thermal contact? Second, thermodynamics can connect various properties of the same material, but cannot explain why various materials have different properties: Mercury expands upon heating, rubber contracts; some materials are good thermal and electric insulators, others are not. The key to the connection between microscopic and macroscopic properties is the entropy. Once it is shown that a certain function of the microscopic states of the system is equivalent to the thermodynamic entropy, the program to explain thermodynamics physically is essentially complete.