This chapter is somewhat different from the other chapters in this text, in that much of the material serves as reference for the following two chapters. We introduce two new models: The Galilean Space-Time Model, which is the basis for developing a useful way of representing variables that are based on spatial dimensions and time. The second is a model of how “things” interact in our physical universe. Forces are the agents of interactions in this model. It is easy to forget that the common and familiar way we talk about distances, speeds, forces and many other variables using these models are not “the way things really are.” They are only this way in the limited range of applicability of these models, which, fortunately, is sufficiently large to include almost all “everyday” phenomena we experience. But whenever we begin to look too closely at the atomic scale, or at systems in which objects travel near the speed of light, or where there are much larger concentrations of matter than we typically experience in our solar system, our familiar notions of space and time and how forces work have to be replaced.