In contrast to predictions or actions, explanations require articulating a model that accounts for the physical phenomenon. Therefore, examination of children's explanations provides a more powerful window onto their developing understanding of causality. This study investigates children's developing causal knowledge, by analyzing changes in the content and form of the explanations they generate, across the age span of three to nine years. The study aims to examine the balance of incremental versus fundamental change and the forms each takes in children coming to understand one physical domain, the working of gears. Thirty-two subjects, ages, 3, 5, 7, and 9, participated in the study. The experimenter elicited each subject's predictions and explanations about what would happen when you turned the knob in a series of gear configurations. Age trends in the explanation type subjects generated revealed broad progress in their understanding of causality within the domain and a complex picture of fundamental and incremental changes. Parsing the sequence of explanation types at points of fundamental change, three phases of development emerged: (a) function of the object as explanation, (b) connections as explanation, and (c) mechanistic explanation. Analysis of development from one phase to the next revealed two forms of fundamental change: radical substitution (where one explanation is supplanted by the next) and transforming incorporation (where one explanation forms the basis for the next and yet is itself transformed in the context of the fundamentally new way of conceptualizing causality). Analysis of development within the individual phases revealed incremental change primarily in the forms of differentiation and decomposition.