This chapter focuses on the role causality play in the development of infants' reasoning. When learning about a physical phenomenon, infants first form a preliminary all-or-nothing concept that captures the essence of the phenomenon but few of its details. In time, this initial concept is progressively elaborated. Infants slowly identify discrete and continuous variable that are relevant to the phenomenon and incorporate this accrued knowledge into their reasoning, resulting in increasingly accurate interpretations and predictions over time. This chapter illustrates the distinction between initial concepts and variables by analyzing experiments on the development of young infants' reasoning about support, collision, and unveiling phenomena.