This paper examines the knowledge construction processes involved when grades 7 and 8 South African students learnt about the concept of gravity while visiting an astronomy-related science centre. The literature on students' understanding of gravity identifies a number of alternative conceptions prevalent, several of which are mirrored in this study. The concept of gravity was demonstrated to learners in a number of ways at the study site, using exposition, models and direct experience of its effects. Using interviews with students, and drawing on a human constructivist framework, the paper shows how knowledge about gravity was acquired incrementally, through subsumption, and more substantially, through greater knowledge restructuring. In addition to students' understanding of the cause of gravity, particular attention is paid to their perception of how it operates on bodies in the solar system. Networks of activity involving the effects of gravity which result in significant knowledge restructuring are recommended as providing the most substantial learning experience in an informal environment such as a science centre.