Over the last 15 years, numeracy has become more and more prominent in curriculum initiatives around the world. Yet, the notion of numeracy is still not well defined, and as such, often not well understood by the teachers who are charged with the responsibility of helping our students to develop their numeracy skills. In this article I explore the work of a team of mathematics teachers brought together for the purpose of developing a set of numeracy tasks for use within district wide numeracy assessments. Results indicate that these teachers' experience designing these tasks, and pilot testing them in their own classrooms, propelled them to make massive changes in their own mathematics teaching practice. Through a lens of Rapid and Profound Change ("Journal of Mathematics Teacher Education" 13:411-423, 2010) the mechanism and catalyst behind these changes are revealed.