In its full development, system dynamics is a discipline with the scope of science, education, law, engineering, or medicine. Many principles form the foundation of system dynamics and become a basis for thinking in all endeavors. Here we touch on the nature of feedback loops. People seldom realize the pervasive existence of feedback loops in controlling everything that changes through time. Most people think in linear, non-feedback terms. System dynamics builds two-way communication between mental models and simulation models. Mental models are the basis for everyday decisions. Mental models contain tremendous stores of information. But the human mind is unreliable in understanding what the available information means in terms of behavior. Computer simulation meshes nicely with mental models by taking the mentally stored information and then displaying the dynamic consequences.