Although simple mechanisms are commonplace, reasoning about how they work— mechanistic reasoning—is often challenging. To foster mechanistic reasoning, we engaged students in the thirdand sixth-grades in the design of kinetic toys that consisted of systems of linked levers. To make the workings of these systems more visible, students participated in forms of activity that we conjectured would afford bodily experience of some of the properties of these mechanisms: constraint and rotary motion. Students progressively re-described and inscribed these embodied experiences as mathematical systems. We report a microgenetic study of one case study student, tracing how embodying and mathematizing motion supported the development of reasoning about how levered systems work.