The authors' school's second-grade science curriculum has always focused on trees and their role in the ecosystem during the first half of the year. Curricular goals have largely emphasized what students ought to "know" by the end of the semester, such as identifying the parts of a tree, describing the process of photosynthesis, and creating a tree guide about trees found in the school yard. In partnership with an NSF-funded research project investigating learning progressions in science inquiry, the authors sought to balance teaching science content with processes and reasoning, and to navigate how to be authentically responsive to children's inquiry without compromising content goals. They worried: What might this new focus do to their curriculum goals? Could they guarantee that the students would learn the science content? In this article, Carla--the school's staff developer--and Diana, second-grade teacher--present one teacher's journey over the course of a school year in navigating these tensions while teaching the "tree unit," with particular emphasis on why the leaves change color. (Contains 1 figure.)