Oh man, that joke made me chuckle, haha. So bad it's good! "cast" as a noun can mean the group of actors who act in a play or a movie. Usage example: "the cast of Hamilton had twenty actors in it", or "Brad Pitt was part of the cast of Ocean's Eleven". However, "cast" as a noun can also mean the white thing a hospital puts on your leg to immobilize it after you break it (google "leg cast" for examples). So it's a play on two of the meanings of the word "cast", since if you break your leg, a cast (the medical type) gets put on it.