Mathematical reasoning is one of the four proficiencies in the Australian Curriculum: Mathematics (AC:M) where it is described as: "[the] capacity for logical thought and actions, such as analysing, proving, evaluating, explaining, inferring, justifying and generalising" (Australian Curriculum, Assessment and Reporting Authority [ACARA], 2017, p. 5); and more recently is spearheading efforts to reform mathematics teaching (Herbert & Bragg, 2017). This article describes ways to incorporate reasoning prompts to explore students' misconceptions, then more specifically present a task with a "truth prompt" that has the potential to afford students with the opportunity to reason mathematically. The authors explain the rationale for the choice of this task and how it was developed into a lesson with potential to assess reasoning. In addition, examples of children's reasoning will be presented and discussed.