How might fourth graders address the problem of measuring the size of a leaf? And why might their classroom teachers choose such a shape to measure? One group of fourth-grade students in an urban Australian school were interested in knowing the sizes of leaves on a passionfruit vine growing at their school. The children were excited after a visit from an agricultural scientist, who shared his knowledge about photosynthesis. This article presents mathematical modeling explored in their classroom, as students explored ways to predict the surface area of a leaf (leaf size), within an agricultural science context involving fruit production. The first author and her co-teacher, who regularly shared teaching responsibilities, worked with an agricultural scientist in designing the inquiry for their fourth-grade class.