This Christmas tradition is only about a decade old.

In 2004, inspiration struck for Carol Aebersold and her daughter, Chanda Bell. For years, their family had enjoyed a Christmas tradition that involved placing a toy elf named Frisbee on a shelf to keep an eye on their kids during the holidays. Kids were told the elf was a spy for Santa Claus, who was much too busy in December to keep an eye on all the world’s children. The duo turned the tradition into a book/toy combo titled The Elf on the Shelf the following year and they began promoting it at trade shows and other events. The book outlines the rules that the elves live by.

In the weeks leading up to Christmas, each elf lives in a family’s home and keeps track of their various activities. Then, after they go to sleep at night, the elves fly back to the North Pole and provide Santa Claus with a full report. The following morning, they pick a different spot in their house to keep watch from. Each elf is “activated with magic” after being named by the family and zooms back to the North Pole after Christmas where they remain until the following year. Also: if anyone touches the elf after they’ve been activated, they’ll lose their magical abilities to keep Santa well informed.

The book has now sold more than six million copies, but there are others who are a bit more critical. A columnist for The Atlantic called the tradition “creepy” and included tales about them driving parents across America crazy as they attempt to come up with new spins on the tradition in order to keep up with the parents of their kids’ friends, or to invent new hiding spots for the elf each night. Some critics have even gone so far as to argue that the elves are preparing American children to eventually “live in a future police state.”