This article examines how conversations during design reviews in which 8th-grade mathematics students shared population models with visiting specialists expanded the disciplinary expertise of the classroom. "Re-contextualizing" is a conversational exchange that visiting specialists initiated to invite groups to consider their models in novel contexts. Analysis of 14 design reviews in 2 classrooms showed that re-contextualizing resulted in both the elaboration of ideas students already understood and new contributions to students' understandings of mathematical aspects of population modeling. This article presents case studies of 2 groups that differed in terms of their interest in the curricular task and the level of conceptual integrity of their population models. Despite these differences, the re-contextualizing exchanges that emerged in their design reviews led to new insights for both groups and provided them with opportunities to try on ways of thinking and acting like population biologists. (Contains 2 tables, 5 figures and 7 footnotes.)