Facebook has rolled out an unannounced treat to mark Harry Potter's 20th anniversary today (June 26th), the day in 1997 when J.K. Rowling first published Harry Potter and the Philosopher's Stone -- which was published the next year in the U.S. as Harry Potter and the Sorcerer's Stone.
If you post the name of any of the Harry Potter houses -- Slytherin, Hufflepuff, Gryffindor or Ravenclaw -- in a post of your own or in a comment, the name of the house will be displayed in its color, along with a wand that shoots out colorful zaps and stars. It also works if you post or comment the name "Harry Potter."