Cooperative Prototyping can be characterized as the use and development of prototypes as catalysts during discussions between designers and potential users -- the overall intention being one of mutual learning. On the one hand, the designers learn more about the work practices of the users in ways that are tied concretely to some current version of the prototype. On the other hand, the users learn more about the potential for change in their work practice, whether computer-based or otherwise. This paper presents the results of a field study of the cooperative prototyping process. The study is based on a fine-grained video-based analysis of a single prototyping session, and focuses on the effects of an open-ended style of interaction between...