The psychologist and developmental biologist Jean Piaget divides cognitive development into four stages: sensorimotor, preoperational, concrete operational, and formal operational. Preschool children fall into the preoperational stage, which spans from toddlerhood -- around 18 to 24 months -- through early childhood, at around seven years old. At this age, children are able to think symbolically, and their language becomes more mature. Their memory and imagination develop, which allows them to play make-believe, as well as to distinguish past from future.
Children in the preoperational stage are still developing their abilities to compare quantities and tend to assume that “bigger” means “more.” For example, consider a child presented with two rows containing the same number of blocks. In one row, the blocks are spaced further apart than the other so that the row appears longer. If the child is asked which row contains more blocks, he will typically choose the longer row, even though both contain the same number.
At around the age of four or five, children are just beginning to make the distinction between public appearances and private thoughts. However, they still tend to be egocentric -- that is, they assume that their private thoughts and point-of-view applies to others. It can be difficult for preschoolers to understand how the world looks to other people. They will often assume that other people are happy if they are happy, hungry if they are hungry, or that other people like whatever they like.
The psychologist and theorist Lev Vygotstky, who worked at around the same time as Piaget, stressed the fundamental importance of social interaction in cognitive development. While Piaget believed that development precedes learning, Vygotsky argued the opposite, that social learning tends to precede and shape cognitive development. Central to Vygotsky’s social development theory is the idea of the “more knowledgeable other,” often abbreviated MKO. The MKO is someone who has a better understanding of a task or concept than the learner. Usually a teacher or adult who interacts with the child, the MKO guides the child from what she can already do independently into the “zone of proximal development” -- the area just outside the child’s current abilities, which represents the area of greatest potential for development.