Whilst spatial reasoning skills have been found to predict mathematical achievement, little is known about how primary (elementary) students' conceptual understanding of three-dimensional objects develops. In this article, we report a qualitative study and the impact of rich learning experiences on 48 Years 3-6 students' geometric reasoning relating to prisms. A one-to-one task-based interview, refined by the researchers, was used to assess student learning. Coding and data analysis were informed by our previous research. The findings reveal noticeable shifts in students' knowledge of and reasoning about prisms, their ability to construct and describe prisms with geometric language, and their visualisation and spatial structuring skills. The implications of these findings highlight the importance of teachers' choice of tasks that require students to compose and decompose three-dimensional (3D) objects; compare 3D objects through physical and mental transformations; take different perspectives; and visualise and reason geometrically.