An elf on the shelf is a toy that comes in a box. It contains an elf, a small book, and a certificate of adoption. The elf is supposed to help parents keep an eye on their children's behavior during the Christmas season. The elf is placed in a different spot in the house every night to watch over the children and report back to Santa Claus. Parents are also encouraged to give their children a special warning or reminder if they have been naughty or nice.