Mathematics education researchers have highlighted the importance of assumptions in school mathematics given their vital roles in mathematical practice. However, there is scarcity of research aiming at enhancing students’ recognition of different roles that assumptions play in mathematical activity. In this paper, we begin to address this issue by formulating two task design principles and reporting on the implementation of a classroom intervention where lower secondary school students in two classes worked, with the same teacher, on a task designed following the proposed principles. Our analysis shows how the task, together with the purposeful teacher’s actions in implementing it, led to students’ developing appreciation of two roles that assumptions play in mathematical activity.