Dynamic systems are all around us - they pervade our lives in addition to the study of the Earth - and many of the pressing questions and problems that we face require a good understanding of how these systems work; how they react to changes.

In this chapter, we have looked at a few simple examples of dynamic systems and have seen how they can be modeled using the program STELLA. By working through the introductory example, you should have acquired a beginning proficiency with the program, which will be used extensively throughout this book. The primary reason for modeling these systems using a computer is that many of them are so complex and so unwieldy as to make experimentation impossible and their dynamics cannot generally be understood without experimentation. STELLA provides a powerful, versatile, and yet user-friendly means for modeling these dynamic systems on a computer.

This chapter has also introduced some of the more common types of system designs and the kinds of behaviors that result from these designs. These basic system structures will occur again and again in a variety of contexts.

One of the major challenges in computer modeling is to understand at what level the results should be interpreted. Should they be taken as precise simulations and representations of the real world? Should their results be considered in a qualitative way only? Should their results be considered only in an abstract sense, without any connection to the real world? Are the results meaningless since the full complexity of nature can never be included in a model? These questions get to the very heart of modeling. Models may be constructed with different intents; some are designed to generate realistic results that have predictive value, while others are merely designed to investigate the implications of some assumption. It is always important to understand what the intent of the model is first - that gives us a clue as to how to interpret the results. Models that attempt to be realistic must be compared with observations of the dynamics of the real-world system.

In almost all cases, making a model requires a good sense for what are appropriate and acceptable simplifications. The water tub system provided a clear example of a range of models that helped to illustrate the difference between "as simple as possible" "not too simple".

With some sense of how to interpret the results of these models, you are prepared to move on and actively explore some fascinating and important aspects of the dynamic system we call Earth.