A day hardly goes by when Facebook news feeds don't include updates from families' "elf on a shelf" – including images of elves wearing sweatbands running through miniature finish line tapes, elves sitting in bathrooms, and elves playing with Legos, among others.

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This photo shows "The Elf on the Shelf: A Christmas Tradition," which includes a picture book and a stuffed, felt elf that has to be moved stealthily every night, traditionally from Thanksgiving until Christmas Eve. After eight years on the market, more than 6 million of the kits, book and elf, have been sold.