The sensorimotor stage is the first stage and lasts from birth to age two. Before we discuss any of Piaget's stages, it is necessary to define some of Piaget's terms, because he invented a unique and sometimes confusing vocabulary. In broad terms, a schema is a way of making sense of experience. In the sensorimotor stage, schemas begin as physical actions and gradually move to a mental level. In simplest terms, the sensorimotor stage is the movement or transformation of schemas from the physical level to the thinking level.
Adaptation is the process of building schemas through direct interaction with the environment.
Assimilation is the part of adaptation in which the external world is interpreted through existing schemas. Accomodation is the part of adaptation in which new schemas are created or old ones are adjusted to produce a better fit with the environment. It is a natural process and occurs automatically. Piaget also used the concepts of equilibrium and disequilibrium. When a child is in a state of equilibrium, there is a high degree of cognitive comfort because he is using assimilation as his primary means of dealing with information.