The episode: “The Painted Hills,” ep. 510
The riff: Morbidly asserted by Tom as the movie’s hero, Lassie the dog (here called “Shep”), appears ready to succumb to poison. Yes, it’s kind of dark.
The explanation: They’re referring to a really well-known series of dog food commercials from the 1970s for a food called “Purina Chuck Wagon.” The commercials all featured a miniature, horse-drawn chuck wagon streaking across a home, chased by a dog. The wagon would go into a kitchen cabinet where Purina Chuck Wagon and its “big meaty chunks” awaited. The commercials were so well known that there was even a game for the Atari 2600 called “Chase the Chuck Wagon” where you played as the dog.
Novelty factor: I’ve never owned a dog and I didn’t live in the 1970s, but somehow I was already familiar with this series of commercials. I honestly don’t remember how. Sometimes even I don’t know why I know things.