Over the last two decades, a significant body of research has documented the nature of intuitive physics knowledge--the knowledge of the world that students bring to the learning of formal physics. However, this research has yet to document the roles played by intuitive physics knowledge in expert physics practice. In this article, I discuss three related questions: (1) What role, if any, does intuitive knowledge play in physics problem solving? (2) How does intuitive physics knowledge change in order to play that role, if at all? (3) When and how do these changes typically occur? In answer to these questions, I attempt to show that intuitive physics knowledge can play a variety of roles in expert problem solving, including some roles that are central and directly connected to equations. This research draws on observations of college students working in pairs to solve physics problems.