It is with great pleasure that we present this book to the public. In principle it is about thermodynamics, especially the simulation of thermo?uid systems. In popular opinion, thermodynamics is considered to be highly abstract and di?cult to comprehend with its many symbols. We endeavor to show the reader how simple and beautiful thermodynamics really is. To achieve this simplicity we apply two innovations: Forus,entropyisasubstance-likeconcept,akindofthermalcharge,analogous to the well-known electric charge, and not the abstract and incomprehensible Clausius integral.