Santa knows when you've been bad or good, so be good for goodness sake, so goes the popular holiday song.
But how does he know it when your family is in Avon and the North Pole is, well, far, far away?
Many Berlin children will tell you it's because of the elf on the shelf.
The growing Christmas trend started after Carol V. Aebersold and her daughter, Chanda A. Bell self-published a book called The Elf on the Shelf in 2005 based on a family tradition. The story, illustrated by Coe Steinwart, is narrated by an elf named Chippey.
Each book comes with an elf that the family names. The elf becomes the "eyes and ears of Santa Claus," traveling every night to report to Santa who's been naughty or nice, according to The Elf on the Shelf website.
"The tradition begins when Santa sends his scout elves out to Elf Adoption Centers," the description on the website states. "Waiting for their families to bring them home, these patient elves hibernate until their family reads The Elf on the Shelf, gives their elf a very special name, and registers their adoption online. Once named, each scout elf will receive its Christmas magic and become a part of the family’s Christmas each and every year."
While the elf is observing the family, he might be found in odd places.
Kids come downstairs one morning to find a message from their elf on the table.
He may be found eating ice cream, reading Christmas books and curled up in a blanket with a cold.
Some elves do more than hide and watch the family. They get a chance to go out on the town, too.
"He was loving the chocolate fountain at Munsons!" said one Berlin mom who brought her elf shopping and posted his photo on Facebook.
Does your family have an elf on the shelf? How long have you had it? Where have you children found your elf hiding?