Looked closely at the development of intelligence in infancy and early childhood. Developed ingenious non-verbal tests to explore young children's conceptions of reality. Saw children constructing reality through the processes of assimilation and accommodation, creating conservations. From about age 2 to 6 or 7. The child reconstructs the world at a symbolic level with language.
Child again reconstructs the world on the basis of rules, classes, numbers and relations.
Child starts to deal with abstractions and how things might be but are not. Looked at memory and imagery. Intelligence develops in a series of age related stages that cannot be hurried. Knowledge is always a construction. It always involves both the mental activity of the child and information from the environment.