Recent discussions in the interaction design community have called attention to sketching as an omnipresent element of any disciplined activity of design, and have pointed out that sketching should be extended beyond the simple creation of a pencil trace on paper. More specifically, the need to deal with all attributes of a user experience, especially the timing, phrasing, and feel of the interaction, has been identified. In this article, we propose extending the concept of sketching with a pencil on paper to the more generic concept of fluent exploration of interactive materials. We define interactive materials as any piece of software or hardware that represents or simulates a part of the interactive user experience, such as input from sensors, output in the form of sound, video, or image, or interaction with Web services or specialized programs. We have implemented the proposed concept within Sketchify, a tool for sketching user interfaces. Sketchify gives designers the freedom to manipulate interactive materials by combining elements of traditional freehand sketching with functional extensions and end-user programming tools, such as spreadsheets and scripting. We have evaluated Sketchify in the education of interaction designers, identifying both successful aspects and aspects that need further improvements.