The growth in science understanding and reasoning of 12 children is being traced through their primary school years. The paper reports findings concerning children's growing understandings of evaporation, and their changing responses to exploration activities, that show the complexity and coherence of learning pathways. Children's responses to identical explorations of flight, separated by two years, are used to explore the interactions between conceptual knowledge and scientific reasoning, and the manner in which they change over this time. The paper discusses the particular insights afforded by a longitudinal study design, and some attendant methodological issues.