Defining better implicit models of children's actions in a series of situations is of paramount importance to understanding how knowledge is constructed. The objective of this study was to analyze the implicit mental models used by children in complex change problems to understand the stability of the models and their evolution with the child's development. The study was structured in two phases. First, a written test was given to 198 fourth-, fifth-, and sixth-graders in order to identify different stable resolution patterns used by children in a set of problems involving the reconstruction of a change. The second phase consisted of individual interviews with 15 children of each level representing all procedures. Three models were identified from the study: sequential model, state comparison model, and change comparison model. Two conceptual leaps in the transition from the different models were identified: the first related to the representation of the problem structure and the other deals with the concept of number. Contains 14 references. (MKR)