Thermodynamics is a core part of science. Nearly all scientists should have a basic knowledge of thermodynamics. Thermodynamics is a science of development, and is a viewpoint on scientific development in natural sciences. The achievement of thermodynamics has influence not only on natural sciences, but also on social sciences and philosophy. Fundamental concepts and definitions are very important for any discipline of science, so what thermodynamics is and what the second law of thermodynamics is have become the key points of puzzlement in the thermodynamics discipline. It has been found that the root of puzzlement is the Carnot theorem. The Carnot theorem emphasized “reversibility”, but the nature of the second law of thermodynamics is “irreversibility”, i.e. “arrow of time”. The extended Carnot theorem emphasizes “nondissipation” and results in a new field of nondissipative thermodynamics, which belongs to the equality part of the second law of thermodynamics as well. Some fundamental concepts of thermodynamics are drastically changed. [1]