A children's game where one person hides their face (or hides) and then suddenly reveals themselves, saying 'peek-a-boo!' to surprise another person, usually a small child.
From 'bo' (an exclamation) + 'peep' (to look quickly or secretly). Popularized by the nursery rhyme about Little Bo-Peep who lost her sheep, dating back to the 1600s.
Peek-a-boo is actually crucial for child development—babies learn 'object permanence' (things still exist when hidden) through this game, which is why it's been played for 400+ years!
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