Invention is an important source of technology, and, to understand invention, one needs a model system or prototype. A candidate model system is the hand tool. Using the hand tool as an example, the authors present a systematic approach to invention, dealing with description, classification, and joining or integrating simpler forms. Heuristics for when to integrate are presented. Finally, the authors introduce a new way of thinking about hand tools: simple spatial transformations applied to an abstract element make possible the (...) re-creation of a large number of tools. (shrink)