The objective of the research was to analyze and understand the development in the explanatory activity on itself and its environment 50 children, ages 6 and 9, who were in first two basic schools from La Serena and Coquimbo, Chile, to contribute characterization of cognitive development. The study methodology was qualitative type, favoring the use of techniques of observation, semi-structured interviews and focus group interviews. They found that children predominantly made theories surrounding elements; they build their theories based on their previous experiences, expressed in a sophisticated way, but are encouraged to reflection can give more complex answers. They use them to predict and explain situations, persuade others, as well as guide your own actions, also showing a tendency to check and test their theories.