Findings from a quasi-experimental study of children's ideas about the solar system and how these ideas changed in response to a 10-week intervention period of formal astronomy teaching at a single primary school in England are presented in detail. Initial interviews with all of the 9- to 11-year-olds involved revealed a relatively poorly developed prior knowledge base, and this was reflected in the predominantly intuitive and transitional nature of the different mental models expressed and used when answering questions and completing tasks. Following intervention, progression was evident in many different forms and this could be described and measured both qualitatively and quantitatively. The routes and pathways toward scientific conceptualization were often direct, and most changes could be attributed largely to the processes of weak and radical knowledge restructuring. Together with the retention of newly formed ideas over time, learning outcomes were considered particularly encouraging. In order to explain findings more fully, evidence is presented which lends some support to the notion of chaos in cognition.