This chapter deals directly with the Energy-Interaction Model. We add all kinds of mechanical interactions to the thermal interactions we treated in Chapter 1. (Note: the term “mechanical” as in the phase “mechanical interactions” is typically used to imply everything other than thermal. Since the Energy-Interaction Model literally applies to every kind of interaction that scientists have ever encountered, we will be just scratching the surface of the realm of applications of this powerful model. We will, however, devote some attention to one area of application that occurs frequently in many phenomena–all kinds of things vibrate, from atoms and molecules to bridges and skyscrapers; that is, they move back and forth or oscillate in very predictable ways.