In its 30-plus years of history, The Simpsons has created books worth of trivia and made a huge imprint on the world of pop culture and entertainment. How well do you know the popular animated series?

Here are 10 facts that only true fans know about The Simpsons!

10) The Simpsons shorts on The Tracey Ullman Show

The Simpsons has its origins in animated shorts that appeared for three seasons on The Tracey Ullman Show on Fox. Nearly 50 of these shorts exist, though they aren't very well known. They aired on Tracey Ullman's talk show between 1987 and 1989.

The Simpsons debut short, "Good Night" (1987)

The Simpsons season 1 debuted later in 1989, which brought an end to the shorts. Ullman continues to get a share of the show's annual profits every year.

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9) Conan O'Brien wrote for The Simpsons

Conan O'Brien, who is now best-known as a late-night talk show host, worked on some of the earliest seasons of The Simpsons. He wrote and produced the show between 1991 and 1993 (seasons 3-4). O'Brien left after he landed his own talk show, Late Night with Conan O'Brien, in 1993. He later appeared in a cameo role in the 1994 episode "Bart Gets Famous."

8) "D'oh!" is in the dictionary

Homer's catchphrase "D'oh!" was added to the Oxford English Dictionary in 2001. It was originally written in the show's script as "annoyed grunt." Homer's voice actor, Dan Castellaneta, says he based the sound on a character from Laurel & Hardy.

7) The "Simpsons" live in Springfield, __, USA?

It's a long-running joke on The Simpsons that the family lives in an unspecified American state. The show's creators chose the city of Springfield because there are dozens of cities with that name in the US. Over the years of the series, the show has given hints, and most believe it is in either Illinois or Oregon.

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6) Character inspirations and names

Many of the show's characters are named after people in creator Matt Groening's life. Groening's parents are named Homer and Margaret, and he has two younger sisters named Lisa and Maggie. His mother's maiden name was Wiggum, which is also used in the show. Bart, meanwhile, was chosen because it is an anagram of "Brat."

Other character surnames come from streets around Groening's hometown of Portland, Oregon. These include Flanders, Quimby, Lovejoy, Terwilliger, and Van Houten.

5) Male children are voiced by women voice actors

It's true! All of the show's male youth are voiced by women actors. "Bart Simpson" is famously voiced by Nancy Cartwright, whose other roles include "Nelson," "Ralph," and "Todd Flanders." Pamela Hayden voices "Milhouse," "Jimbo," and "Rod Flanders."

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4) The "McBain" clips form a short film

In The Simpsons' earlier seasons, there are numerous clips seen from an action movie parody called "McBain" (played by the show's Arnold Schwarzenegger parody, "Rainier Wolfcastle"). Fans later discovered that if the clips from many episodes are pieced together, they form a short film.

3) Bart's prank calls to Moe had a real-life inspiration

In the early seasons of the show, a popular gag was "Bart's" prank phone calls to "Moe, the bartender." "Bart" would provide a fake name that "Moe" would call out, but it would be a pun or offensive homophone. 

The joke was actually based on a true story of calls that were placed to the bartender and former boxer Louis "Red" Deutsch in the 1970s.

2) Waylon Smithers was black in season 1

In season 1 of The Simpsons, "Waylon Smithers" had black skin and blue hair. Every season thereafter, he had yellow skin and grey hair. Matt Groening explained the reason for the change: "He was always yellow, and then they painted him wrong once. That's all… At the time, we didn’t have enough money to do retakes, so when there were glitches and mistakes, they stayed that way."

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1) "Krusty the Clown" was supposed to be Homer's secret identity

If you've ever thought "Krusty the Clown" looks like "Homer" in make-up, that wasn't a mistake. "Krusty" was designed with the thought that "Bart" would worship him and later learn that "Homer" secretly portrayed the TV clown. This idea was soon abandoned as it was too complex. "Krusty" instead gained a backstory of his own.

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