In after-school programs, skill-building is a holistic process by which adolescents--guided by adults--achieve mastery. Developmental theories such as Bronfenbrenner's bioecological model position youth as active learners; however, little is known about the specific actions youth use to enhance their learning during skill-building opportunities. Qualitative analysis of 49 semi-structured observations of enrichment activities at a high-quality after-school program showed that adolescents used four types of actions while completing a project: inquiry, contribution, self-regulation, and peer education. These behaviors indicated youths' level of cognitive engagement. Instructors' practices related to questioning, monitoring, group management, and sharing control demonstrate the range of instructional practices that can be used in response to teens' use of the four types of learning actions. This study presents a theoretical model of the skill-building process that illustrates how teens' behaviors interact with staff practices and the demands of project-based learning.