As of 2019 there are 30 "Treehouse of Horror" episodes, with one airing every year. They are known for being more violent than an average "Simpsons" episode and contain several different trademarks, including the alien characters Kang and Kodos who have appeared in every episode. Quite often the segments will parody well-known films, books, radio shows, and television shows. "The Twilight Zone" has been parodied quite often and has been the inspiration for many segments. The 31st season made a Thanksgiving themed episode named "Thanksgiving of Horror".