Substantial resources are directed toward public engagement by the physics community, but there is still much to understand about physicists' initial motivation for involvement, their experiences and their reasons for sustained participation. We use motivation theory as a lens to investigate the factors that contribute to physics students' decisions to participate in informal science programs. We conducted surveys and interviews with university students that volunteered in an afterschool

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... program for K-12 students. Interviews were analyzed using content analysis. Findings indicate that previous participation in informal activities, attitudes towards engaging with the community, and recognition from peers are common themes in university students' initial reasons for involvement. The emergent themes of a positive experience and the enjoyment of sharing physics are related to students' persistence in the program. edited by Jones, Ding, and Traxler; Peer-reviewed,