This paper investigates (digital) materiality through an analysis of the “sociomaterial configuration” (Orlikowski 2009) of the participatory design project SensorLab (2010). In SensorLab, users were enrolled as designers: a group of high school students developed and tested smart pollution-sensing prototypes in a public park in Amsterdam. Concepts from science and technology studies, specifically the notion of the “dance of agency” (Pickering 1995), are used to trace how ‘smartness’ materialises in the form of the SensorLab’s prototypes. The exploratory case study draws conclusions about (1) how materiality performs its agency and invites improvisations during prototype design and (2) how the student-designers use their tacit knowledge as situated expertise to improvise with construction materials and technology. The deconstruction of the assemblage of human/material agency suggests that while the student- designers are readily accommodated to develop prototypes, the material agency of the sensor technology resists improvisation as compared with the other available materials. The extent to which the black-boxed sensor technology allows the student-designers to become ‘smart’ is therefore debatable.