Peekaboo (also spelled Peek-a-boo, also called Peep-eye in the Southeast US) is a game similar to hide and seek, but played with babies. In the game, one (child, teenager, or adult) hides their face, pops back into the baby's view, and says — to the baby's amusement — Peekaboo! I see you! Peekaboo is thought by developmental psychologists to demonstrate an infant's inability to understand object permanence. Object permanence is an important stage of cognitive development 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 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, which explains why they do not cry when their mothers are gone. "Out of sight, out of mind." A lack of Object Permanence can lead to A-not-B errors, where children reach for a thing at a place where it should not be.
