Cognitive-developmental theories focus on how thinking processes change, qualitatively, with age and experience. A classic cognitive-developmental theory is that of Swiss psychologist Jean Piaget, who conducted innumerable research studies beginning in 1920s and continuing through the 1970s. Piaget portrayed children as active and motivated learners who interact with and increasingly adapt to their physical and social worlds through two processes; assimilation (responding to and possibly interpreting an object or event in a way that’s consistent with an existing scheme) and accommodation (either modifying an existing scheme or forming a new one in order to deal with a new object or event). Piaget proposed that children’s cognitive development is to some degree propelled by the process of equilibration: Children encounter situations for which their current knowledge and skills are inadequate *such situations create disequilibrium) and may be spurred to acquire new knowledge and skills that help return them to a state of equilibrium.