A range of challenging activities centred on evaporation and condensation were explored with year 1 and year 6 children. Their explanations in group discussion, written responses and interview were analysed, using NUD*IST qualitative analysis software, to explore the nature and coherence of their conceptions. The data was used to critically evaluate previous claims in the literature concerning the characteristics of progression in understanding of evaporation, and to identify the main dimensions that characterize the differences in children's explanations. The older children displayed a surer sense of ontological categories, greater epistemological sophistication including their ability to link explanations and evidence, greater precision in the use of conceptual language, and a greater range of associations they could make.