Several experiments with 8- to 9-year-old children are reported to demonstrate that the décalage observed between success in problems of conservation of weight, volume and density is due to the different task situations as presented by Piaget and Inhelder. When the tasks are rendered parallel, the traditional décalage disappears. A superior level of conservation of volume was found in 12- to 15-year-old adolescents. The explanation of the observed phenomena is to be sought in Piaget and Garcia’s conception of a dialectical interaction between logical operations and notions of physical causality.