Ever since it debuted in the summer of 1997, South Park has been an instant classic that won adult audiences with its crude humor, hilarious catch phrases, and themes that no one before has even dared to explore. Now, in honor of its twentieth anniversary, here are 10 things you probably didn’t know about the show.

 It paved the way for more mature content

It wasn’t easy for this show to even premiere on television. Luckily, the same year it aired, the TV parental guidelines were created and the rating system was used to distinguish adult content from child-friendly content. And that is how South Park got the first ever “mature audiences” label and was able to make audiences everywhere watch a cartoon made for adults

 Female cast members of the show are often credited under pseudonyms

Most of the male characters are voiced by the creators themselves, T Ray Parker and Matt Stone. However, the females of the fictional town were voiced by a small number of women that worked behind the scenes.

 Several celebrities have guest starred, but not in the roles you would have expected

Every fan of the show will tell you that South Park often makes fun of various celebrities that are poorly voiced on purpose. But when a celebrity wants to lend their voice to the animated show, they are usually given a smaller part, sometimes even only animal sounds. It’s hard to believe that some big-name celebrities agreed to this, but it’s true. For example, George Clooney provided dog barks for the show, Jay Leno was a purring cat, and Henry Winkler was a growling monster.

 One very famous fan of the show was offered a role, but he declined it

Not everyone wants to be on the show so badly that they would be prepared to voice an animal. When Jerry Seinfeld contacted the creators of the show, asking for a guest role, he was offered a non-speaking part. They asked him if he would be interested in voicing a turkey in their Thanksgiving episode, but Seinfeld declined the offer.

 Some famous names have written for the show before they had their big breaks

When you think of Kristen Schaal and Bill Hader today, you probably think of them as the comedy stars they are today. But back in the day, while Hader was still on SNL, he was a consultant writer and later producer for South Park. And as for Schaal, she was also a consultant writer way back in 2007 before her career took off and she landed roles on The Daily Show and 30 Rock.

 Two episodes had a large contribution from a sitcom legend

Tray and Parker are known for their daring and controversial humor and that ultimately was what attracted the creator of the most politically incorrect sitcom, All in the family, Norman Lear. When he decided to bring his talents to South Park, two of the best episodes were born. The 100th episode or of the series, “Canceled” and “I’m a little bit country” were both created by this legend.

 Aliens

Aliens have featured in every episode of South Park, but sometimes, they were a bit more inconspicuous than other times. Of course, the pilot episode saw the alien abduction of one of the boys, Eric Carman. Ever since then, every episode has featured aliens in one way or another, and not only in the introduction scenes.

In some episodes, they play vital roles like in the pilot. In some, they’re casually standing in the background and sometimes, they appear behind tinted windows. Therefore, keep an eye out for these giant-eyed invaders.

 The show reunited a very famous duo

In a season four episode that aired in the year 2000, “Cherokee hair tampons”, two comedians who had long ago finished their partnership, Cheech Marin and Tommy Chong, had guest roles. Even though they didn’t record their parts together, both of them say that their appearances on the show were what revived their professional partnership that ended in the 90s. Since then, they’ve appeared in a number of television shows and movies together.

 An insect mutation got its nickname from a character from South Park

During the first five seasons, Kenny McCormick was the unluckiest kid in the world: he died in every episode but was resurrected in the next… only to die again. And this trait is what led to a mutation in the structure of a fruit fly to be named after him. A gene that was found in 2002 that predicts mortality was nicknamed “Kenny.”

 The show was praised for its portrayal of Tourette syndrome

When a season 11 episode that focused on the syndrome premiered, the Tourette Syndrome Association expected to be offended because the show’s history is a bit crude and it was hard to approach such a sensitive topic and give it justice. However, the South Park creators actually managed it well. The episode’s heavy focus on involuntarily cursing, which is one of the symptoms of the disease, was portrayed perfectly. And for this, the association actually praised the episode for the proper treatment and research of the disease.