Hibbert's jiggle-test. Hell, he apparently has an unprecedented level of body fat based on Dr. However, when Milhouse's father was later introduced, Hank Azaria tried doing a Milhouse-esque voice and the producers thought it was hilarious. It's not slim 'n slender, but it's not the cartoonish level of obesity that defines Homer in the show.
1. Mr. and Mrs. Van Houten Are Clearly Related
Why do the Simpsons never call up Unkie Herb and ask for money? We like you. In episode 2F09, when Itchy plays Scratchy's skeleton like a xylophone, he strikes the same rib in succession, yet he produces two clearly different tones. Kirk and Luann Van Houten both look terrifyingly similar, to such a specific degree that there's almost no question they're related in the show's universe.