Designing usable smart products is a multidisciplinary effort that requires people with very diverse knowledge to collaborate. This work explores how Design mock-ups are used to facilitate collaboration. The literature suggests the analytical boundary object concept as a theory to explain how mock-ups support the integration of knowledge in design collaboration. Supported by data collected during a one year product development project, I discuss the limitations of the boundary object concept and suggest an alternative concept that better explains how mock-ups act as a scaffold for the creation of a shared understanding.