10 Genius In-Jokes In The Simpsons

Fan favourite season four episode "Homer the Heretic" features a gag you might've seen dozens of times and never really understood - but laughed anyway.

While Homer is singing Tom Jones' "Delilah" in the shower, we linger briefly on a shower radio bearing the label "No soap-radio."

While the label seems to be a reference to the fact that the radio is waterproof, being in the shower and all, it's actually a reference to the surreal practical joke known as "no soap radio."

The prank involves telling a joke where the punchline has no relation to the setup, with the joke-teller and bystanders in on the joke both acting like it's hilarious and ridiculing the recipient for seemingly failing to get it.

At its core the no soap radio is an experiment to challenge social norms and particularly the notion of conformity, given how many targets of the prank will start laughing simply in order to not appear stupid - or so they think.

Even today no soap radio isn't a widely-known gag, but back in the pre-Internet days of 1992 when this episode was released? You were a real smart cookie if you caught it unprompted.