Writing a lab report can be an opportunity for students to engage in scientific thinking. Yet students' lab reports often do not exhibit evidence of such engagement. Students' writing can appear focused on "filling in" required components and reporting on predetermined conclusions. We conducted a design experiment in an introductory biology laboratory course and examined the impact on students' engagement in argumentation in lab reports. Over two design iterations, students' arguments more often considered and integrated multiple claims, included a broader range of evidence and ideas, and gave appropriate attention to uncertainty in conclusions. We argue that two interrelated changes to the design of the lab course made these shifts possible. First, we restructured the role of instructors to position them as an audience interested in students' thinking. Second, we introduced more uncertainty into the lab activities to provoke consideration of multiple interpretations. We propose that these changes created a different "rhetorical context" that helped motivate and shape students' engagement in argumentation. More broadly, we suggest that an important alternative to explicitly scaffolding knowledge and skills is to design learning environments that can inspire students to engage in a range of scientific practices more authentically.