We describe the main principles of the framework theory approach to conceptual change and briefly report on the results of a text comprehension study that investigated some of the hypotheses that derive from it. We claim that children construct a naive physics which is based on observation in the context of lay culture and which forms a relatively coherent conceptual system--i.e., a framework theory--that can be used as a basis for explanation and prediction of everyday phenomena. Learning science requires fundamental ontological, epistemological, and representational changes in naive physics. These conceptual changes take a long time to be achieved, giving rise to fragmentation and synthetic conceptions. We also argue that both fragmentation and synthetic conceptions can be explained to result from learners' attempts assimilate scientific information into their existing but incompatible naive physics.