In this article, we describe a preliminary study that integrates research on engineering design activities for K-12 students with work on microworlds as learning tools. Here, we extend these bodies of research by exploring whether—and how—authentic recreations of engineering practices can help students develop conceptual understanding of physics. We focus on the design–build–test (DBT) cycle used by professional engineers in simulation-based rapid modeling. In this experiment, middle-school students worked for 10 hr during a single weekend to solve engineering design challenges using SodaConstructor, a Java-based microworld, as a simulation environment. As a result of the experiment, students learned about center of mass. Our data further suggest that in the process of simulation-based modeling, rapid iterations of the DBT cycle progressively linked students' interest in the design activities and understanding of the concept of center of mass. We suggest that these rapid iterations of the DBT cycle functioned as exploratoids: short fragments of exploratory action in a microworld that cumulatively develop interest in and understanding of important scientific concepts. © 2006 Wiley Periodicals, Inc. J Res Sci Teach