The development of children's cosmologies was investigated over a 13-year period, using multi-modal, in-depth interviews with 686 children (217 boys, 227 girls from New Zealand and 129 boys, 113 girls from China), aged 2-18. Children were interviewed while they "observed" the apparent motion of the Sun and Moon, and other features of the Earth; "drew" their ideas of the shape and motion of the Earth, Moon and Sun, and the causes of daytime and night-time; then "modelled" them using play-dough; which led into "discussion" of related ideas. These interviews revealed that children's cosmologies were far richer than previously thought and surprisingly similar in developmental trends across the two cultures. There was persuasive evidence of three types of conceptual change: a long-term process (over years) similar to weak restructuring; a medium-term process (over months) akin to radical restructuring; and a dynamic form of conceptual crystallisation (often in seconds) whereby previously unconnected/conflicting concepts gel to bring new meaning to previously isolated ideas. The interview technique enabled the researchers to ascertain children's concepts from intuitive, cultural, and scientific levels. The evidence supports the argument that children have coherent cosmologies that they actively create to make sense of the world rather than fragmented, incoherent "knowledge-in-pieces". (Contains 6 tables and 5 figures.)