Science internships where students work with scientists have been suggested to have many positive impacts on students' science learning. However, little research has been conducted to investigate the types of interactions that are beneficial for the development of science knowledge through an authentic internship experience. The purpose of this study was to illustrate the key features of dynamic interactions and activities involved in an open-inquiry-based internship programme for high school students. Drawing on cultural-historical activity theory, we aimed to describe the features of the internship activity system in terms of the moments of "subject," "object," "tools," "community," "rules," "division of labour," and "outcome." Our analysis suggests that the activity system of the university internship has unique features that promote optimal science learning opportunities. The implications of these unique features are discussed and suggestions are made to improve K-12 science education.