Mathematical conventions -- which account for the choices of mathematics community regarding definitions of concepts, their names and symbols -- are the focus of this paper. We introduce the task of unpacking, that is, offering plausible explanations and arguments for the choice of conventions. Using responses of four prospective teachers to the task we illustrate that unpacking conventions can lead learners to active engagement with mathematical concepts. We conclude by drawing avenues for the use of convention-unpacking tasks in research and practice.