  Under the category of heuristic models, simulations can be further subdivided into those used to communicate knowledge to others, and those used to represent information to ourselves. When Watson and Crick played with tin plates and wire, they were doing the latter at first, and the former when they showed the results to others. When the army corps built the model of the San Francisco Bay to convince the voting population that a particular intervention was dangerous, they were using it for this kind of heuristic purpose. Computer simulations can be used for both of these kinds of purposes—to explore features of possible representational structures; or to communicate knowledge to others. For example: computer simulations of natural processes, such as bacterial reproduction, tectonic shifting, chemical reactions, and evolution have all been used in classroom settings to help students visualize hidden structure in phenomena and processes that are impractical, impossible, or costly to illustrate in a “wet” laboratory setting.