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Elf on the Shelf is a popular Christmas tradition that began in the 1970s. It originated in Finland, where children would hide an elf in their home during Advent, the period leading up to Christmas. The elf would report to Santa Claus every night, who would then reward the children for their good behavior. The tradition has since become popular in the United States and is a beloved part of many families' holiday celebrations. Elf on the Shelf has also expanded to include books, plush dolls, and other merchandise.