Reasoning about action is central to much of intelligent behavior. Much of the reasoning that is needed consists not of reasoning about long or complicated sets of actions, but of thinking of a problem at various different levels of abstraction. Problems that in one representation seem enormous can be reduced to a simple exercise in a more suitable representation. A transformation that is especially useful is one from continuous to discrete change. The transformations necessary to move from a complex continuous system to a simple discrete systems involve both introducing new properties that change with time--fluent names--or no longer modeling some other fluent names, and considering new composite actions.