In this paper we propose the concept of ’active’ and ’passive’ physicality as mental models to help in understanding the role of low fidelity prototypes in the design process for computer embedded products. We define ‘active physicality’ as how the prototype and its software react to users and ‘passive physicality’ as how the prototype looks and feels offline. User trials of four different types of ‘low fidelity’ prototypes were undertaken using an existing product as the datum. Each prototype was analysed in terms of active and passive physicality and user responses were collated and compared qualitatively and quantitatively. The results suggest that prototypes that balance both active and passive physicality produce data closer to the final device than those that are strong in one at the expense of the other.