This paper contributes to the ongoing effort to create rich learning opportunities for prospective teachers to engage with reasoning and proving. Twenty elementary and middle school pre-service teachers completed individual projects in which they explored "math-tricks" – unconventional computational algorithms – as a part of an undergraduate proof course. Our findings suggest that the task evoked uncertainty with respect to why the tricks work and motivation to resolve the uncertainty by means of algebraic proof. We discuss the potential of this task to create rich opportunities for prospective teachers to conduct explorations, construct algebraic proofs and reflect on their experience from learner's perspective.