Modeling is being used in teaching learning science in a number of ways. It will be considered here as a process whereby children of primary school age exercise their capacity of organizing recognizable and manageable forms during their understanding of complex phenomenologies. The aim of this work is to characterize this process in relation to the modeling of properties of and changes in materials. The data are discussed by establishing relationships between the modeling process with three different aspects: the specialized scientific knowledge, the physical manipulation of phenomena, and the interaction among those participating in the class. The results show how 7-8-year-old students generate a modeling process that leads them to explain the behavior of different materials by using a "model of parts" created ad hoc. This model, built up from some kind of a discrete vision of the material, proves to be coherent for children of this age and evolves by relating the visible continuum with an imagined discontinuum.