The aim of this study was to shed some light on the organization of naive knowledge, and on the process of conceptual change in everyday physics, more specifically regarding the concepts of sound and heat. Eighty-three 8-year-old children were interviewed individually in order to see if they attributed the properties of objects (such as substantiality, weight, permanence, and trajectory) or the properties of physical processes (such as transmission by adjacency) to sound and heat. The results indicated that material properties were attributed to these concepts in a hierarchical way. Permanence was abandoned first, then weight, and finally substantiality. The properties of matter were attributed somewhat more for heat than for sound. For substantiality, five different mental models constrained by different naive theories were inferred from the children's arguments. Naive knowledge about sound and heat thus appears to be organized in a relatively coherent way and conceptual change from one naive theory to the next seems to be slow and gradual.