The question motivating this chapter is: What are the key characteristics of double stimulation and what is its role in concept formation in an everyday work activity facing transformations? The analysis shows how volitional change actions emerge and take shape in processes of double stimulation in critical encounters, and how they lead to attempts at concept formation. Critical encounters are fruitful breeding grounds for double stimulation and concept formation efforts in work activities. They may be described with the help of two dimensions, namely the dimension of restrictive vs. expansive use of artifacts and the dimension of incidental vs. planned use of artifacts. Conceptualization efforts often accompany the volitional actions resulting from the expansive use of artifacts. These conceptualization efforts are seldom conscious attempts to fully define or explain a concept. The conceptualization efforts are predominantly fragmentary and focused on specific aspects of the emerging concept. In this light, conceptualization efforts in critical encounters resemble the construction of a mosaic pattern distributed in time and social space across many actors and encounters. Although seemingly modest and piecemeal, such efforts may lead to a radical sea change over time.