The episode: “A Date with your Family,” the short in front of “Invasion, U.S.A.,” ep. 602
The riff: Said in a concerned voice by Mike as the family’s eldest boy checks on dinner in the oven.
The explanation: This is just a morbid joke about the death of American poet and novelist Sylvia Plath. Known for her dark outlook in fiction and poetry, Plath battled depression for most of her life and finally took her own life at age 30. She committed suicide by sticking her head in the oven and died of carbon monoxide poisoning. As is clear here, the writers of MST3k did not shy away from these kinds of subjects.
Novelty factor: I honestly don’t know much about Plath’s own work, but I still understood the reference. This is one of those cases where the manner of a famous person’s death makes it remembered far more widely than it would have been otherwise.