Children gradually comprehend object permanence - the realization that objects exist even when the items are placed out of sight.
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Mead and Role-Taking

Role-taking often occurs through play and games, as children try out different roles (such as being mommy, daddy, doctor, teacher) and gain an appreciation of them.... Then the person begins to construct his or her own roles (role-making) and to anticipate other individuals' responses. Finally, the person plays at her or his particular role (role-playing).

-Diana Kendall, Sociology in Our Times