Ahead of Christmas, the Elf on the Shelf challenge has taken social media by storm. It began with Hollywood actor Garcelle posting a picture of her sitting on Pharell Williams’s shoulder. Soon enough, several other stars joined in.

Elf on the shelf is a popular holiday tradition, in which parents place a small elf doll in their home and tell their children that the elf watches over them during the day and reports back to the North Pole at night. Each morning, after returning from his "mission", the elf is placed in a different location in the home. This year, celebrities are sharing images of themselves with another celebrity super-imposed onto their shoulder. The catch: Like 'elf on the shelf,' their names must rhyme.