While spatial reasoning skills have been found to predict mathematical achievement, little is known about how primary students' conceptual understanding of three-dimensional objects develops. This paper reports insights into a Year 4 student's spatial reasoning when constructing and describing the properties of rectangular prisms, using Froebel's Gifts, in an interview. Categories of the van Hiele framework were used to analyse student responses in conjunction with a specially designed analysis tool. The findings highlight the benefit of using a one-to-one interview to shed light on one student's spatial reasoning and conceptual knowledge of rectangular prisms.