The sensor motor stage
The sensor motor stage is the earliest period because it involves seeing, hearing, moving, touching, and tasting, smelling and so on. The infant at this stage develops object permanence which is a key development at this stage, further they develop goal-direct actions.
The preoperational stage
The preoperational stage is the second stage which the child starts to learn to reverse actions. At the first stage (sensorimotor stage) this will have just started. But at this stage it is mastered. Children develop the semiotic function which involves working with named symbols and pictures to represent real ones. Also reversible thinking which enables children to reverse or to think back or reverse steps in a task is learnt. Piaget informs that at the preoperational stage, children are egocentric which means to tell that they have their own world and the experiences of others from their own viewpoint. It does not mean selfish. They assume everyone else shares their feelings, reactions and perspectives.