The purpose of this study is to identify features of the knowledge-building processes that secondary science students spontaneously develop and consider useful in the context of personal and social action. Capabilities involved in building things (i.e., in creating physical artifacts that embody functional understanding in a novel and physically contextualized way) are an important source of learning and knowing. Our research focuses on questions of how students create meanings for themselves in complex and evolving building environments. Our interpretations are grounded in a perspective on cognition that is constructivist (i.e., students create their own meanings for their experiences) and enactivist (i.e., person and environment are mutually specified). Problem settings that allow for a high degree of student self-direction permit multiple viable paths of action-thought to be created that contribute to students' understanding of their capabilities. Four examples of students engaged in self-directed building projects are presented in the form of interpretive vignettes to illustrate a progression of increasing scope and depth in the context of the building activity and student learning outcomes. We were present every day during the classroom building projects to help, videotape, and interview students. An out-ofschool component was also studied in two of the projects. Our unit of analysis is personacting-in-a-setting. The first example focuses on Amy's metacognitive awareness that thought and action in building have strategic properties. Ronnie and Willie discover a pathway to success through individual and social (classroom) action. Ian develops skill in managing a complex set of social factors in a project with a large out-of-school social context. Dan reflects on the development of his learning in a sequence of self-directed building engagements over a period of 3 years. The enactivist ideas of enaction, coemergence, mutual specification, and adequate conduct are used to describe and interpret the four cases. Each case illustrates ways in which personally significant meanings and personal competency emerge within the context of self-directed action-thought.