Peekaboo is a game played with babies. In the game, the older player hides his/her face, pops back into the baby's view, and says Peekaboo! sometimes followed by I see you!

Peekaboo is thought by developmental psychologists

Developmental psychology

Developmental psychology, also known as human development, is the scientific study of systematic psychological changes, emotional changes, and perception changes that occur in human beings over the course of their life span. Originally concerned with infants and children, the field has expanded to...

 to demonstrate an infant's inability to understand object permanence. Object permanence

Object permanence

Object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, or touched. It is acquired by human infants between 8 and 12 months of age via the process of logical induction to help them develop secondary schemes in their sensori-motor coordination...

 is an important stage of cognitive development

Cognitive development

Cognitive development is a field of study in neuroscience and psychology focusing on a child's development in terms of information processing, conceptual resources, perceptual skill, language learning, and other aspects of brain development and cognitive psychology compared to an adult's point of...

 for infants. Numerous tests regarding it have been done, usually involving a toy, and a crude barrier which is placed in front of the toy, and then removed, repeatedly. In early sensorimotor stages, the infant is completely unable to comprehend object permanence. Psychologist Jean Piaget

Jean Piaget

Jean Piaget was a French-speaking Swiss developmental psychologist and philosopher known for his epistemological studies with children. His theory of cognitive development and epistemological view are together called "genetic epistemology"....

 conducted experiments with infants which led him to conclude that this awareness was typically achieved at eight to nine months of age. Infants before this age are too young to understand object permanence. A lack of Object Permanence can lead to A-not-B error

A-not-B error

A-not-B error is a phenomenon uncovered by the work of Jean Piaget in his theory of cognitive development of children. The A-not-B error is a particular error made by infants during substage 4 of their sensorimotor stage.A typical task goes like this: An experimenter hides an attractive toy under...

s, where children reach for a thing at a place where it should not be.