This study addresses how to help elementary science teachers explore the uncertainty inherent in scientific activity and support elementary students to engage in more complex and authentic investigations. We describe a district partnership focused on understanding how to support elementary teachers to adapt curricula to promote science practices. We then present a close analysis of how teachers navigated ideas about uncertainty during their work exploring tools to adapt their curriculum. We argue that an essential aspect of the teachers' work was developing a more nuanced view of scientific uncertainty, including more precise goals for students' engagement in scientific activity and a repertoire of strategies for supporting "students" to engage with scientific uncertainty without unduly increasing uncertainty "for teachers." We trace three strategies that appeared to help teachers negotiate and develop this more nuanced view: beginning with complex phenomena, iterating on investigations, and leveraging variability in students' ways of conducting investigations. The findings have implications for the design of professional development programs for elementary teachers, particularly the support that teachers might need to negotiate a nuanced set of teacher and student roles when seeking to engage students more authentically in science practices.