In this article we draw on ideas of boundary crossing and boundary objects as a way of thinking about preservice teachers’ metaphors about the nature of mathematics. During a graduate primary mathematics education paper, the writing of metaphors was employed as a reflective tool to support preservice teachers to conceptualise their beliefs about the nature of mathematics, and to consider implications for their practice as teachers. Focus-group discussions were also held. The metaphors, and focus-group data, were analysed in light of literature about boundary objects and Akkerman and Bakker’s (2011) “mechanisms of learning” at the boundary. We suggest that considering metaphors as conceptual boundary objects provides a lens with which to view preservice teachers’ beliefs about the nature of mathematics. A deeper understanding of how we could work more effectively with preservice teachers’ beliefs about the nature of mathematics has emerged. It is suggested that the action of writing, reflecting, and sharing metaphors has the potential to assist both preservice teachers and teacher educators to recognise and work towards boundary crossing. This is a reciprocal process that results in learning for all concerned.