Prototypes are at the core of many interaction design projects. They not only allow designers to formally evaluate their design, but also to explore their project's design space, generate ideas, and discover new design opportunities. This requires the designer to engage in a reflective conversation with her prototypes - closely listening for feedback, combining ideas, and discovering new qualities. This thesis analyzes the prototypes developed for an outdoor light installation, the Urban Data Posts, in order to gain a better understanding of the interplay between designer and prototype. Using a framework to track intentionality and unintentionality in prototypes, this thesis aims to understand how individual prototypes informed the design process and the final design of the Urban Data Posts project. This will provide a better understanding of what prototypes do and exemplify what impact unintentionality in prototypes can have in design. The main contribution of this research is the Focus Framework, which captures intentional as well as unintentional design aspects in a prototype.