In this paper we describe visitor interaction with an interactive tabletop exhibit on evolution that we designed for use in natural history museums. We video recorded 30 families using the exhibit at the Harvard Museum of Natural History. We also observed an additional 50 social groups interacting with the exhibit without video recording. The goal of this research is to explore ways to develop "successful" interactive tabletop exhibits for museums. To determine criteria for success in this context, we borrow the concept of Active Prolonged Engagement (APE) from the science museum literature. Research on APE sets a high standard for visitor engagement and learning, and it offers a number of useful concepts and measures for research on interactive surfaces in the wild. In this paper we adapt and expand on these measures and apply them to our tabletop exhibit. Our results show that visitor groups collaborated effectively and engaged in focused, on-topic discussion for prolonged periods of time. To understand these results, we analyze visitor conversation at the exhibit. Our analysis suggests that social practices of game play contributed substantially to visitor collaboration and engagement with the exhibit.