What is object permanence and at what age does it develop?

Object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, or touched. It is a key milestone in Piaget's sensorimotor stage. Before developing object permanence, infants act as if an object that disappears from view no longer exists, showing no attempt to search for it. Object permanence typically begins developing around eight months of age and is fully established by about eighteen to twenty-four months. Piaget tested it by hiding toys under blankets and observing whether infants searched for them.