In this article, the authors discuss "function," a well-defined rule that relates inputs to outputs. They have found that by using the input-output definition of "function," they can examine transformations of functions simply by looking at changes to input or output and the respective changes to the graph. Applying transformations to the input and output values and examining the resulting graphs helps students build a better understanding of functions. Most important, this technique allows students to keep their intuitive notions of number and operations without having to memorize counterintuitive procedures. (Contains 8 figures and 1 table.)