Is it ever possible to truly grow up? Adults will inevitably tell you there are moments when they feel like frauds -- mere children disguised as grown-ups. And maybe that's the reason there are so many children's television series with considerable crossover appeal. Animated or live-action, these shows may be marketed to kids, but they possess a certain mature quality that brings in an adult fan base too.

It's a challenge for the creative people who make TV for kids: How do you entertain the youngsters while keeping the tone sophisticated enough to prevent adults from bolting out of the room? Sometimes, a show can lean so far in the direction of amusing the parents that its "for kids" classification feels inadequate. These are the TV series that tend to fly straight over children's heads and right into the brains and hearts of would-be grown-ups.

'The Ren & Stimpy Show' (1991 - 1995)

One of Nickelodeon's three pilot NickToons, "Ren & Stimpy" embedded the charming phrase "You eeediot!" into the minds of an entire generation. Looking back, it seems impossible that the show aired in the cold light of day on a children's network. That's how dark, graphic, and sometimes perverse its jokes could be. Ren the chihuahua and Stimpy the cat became icons of subversive animation during their time in the spotlight. Admit it: You still know every beat of the "Happy Happy Joy Joy" song.

'Animaniacs' (1993 - 1998)

"Animaniacs" packed as many pop culture and classic Hollywood references as it possibly could into any given episode, pulling most greedily from the treasure trove of its Warner Bros. Studio pedigree. That's what you get when you put mega-director Steven Spielberg in charge of a cartoon. While the kids sang along to Yakko, Wakko, and Dot's bouncy and sometimes educational songs, the big kids were appreciating the show's pitch-perfect "Goodfellas" parody, "The Goodfeathers," and sneakily off-color jokes.

'The Adventures of Pete & Pete' (1991 - 1996)

"The Adventures of Pete & Pete" is another late 20th century Nickelodeon gem, hailing from the same pleasantly weird era for kids' entertainment as some other shows on this list. The live-action comedy had a heavily surrealist bent, making it one of the most unique and creative shows to appear on the network -- Petunia the dancing tattoo! Mr. Tastee, the ice cream man who never took off his cone head! The bizarre suburban adventures of two identically named brothers got the hipster seal of approval from guest stars like Janeane Garofalo, Luscious Jackson, Patty Hearst, and LL Cool J,and even boasted a suitably alt-rock "house band" called Polaris.

'Adventure Time' (2010 - )

Walk into any Comic-Con in any city, and you'll instantly spot attendees of all ages "cosplaying" the lead duo of "Adventure Time," best friends Jake and Finn. The Cartoon Network series has earned praise and awards for its inventiveness and sense of wonder, and its audience is a devoted one. The show's unexpectedly deep themes are exemplified by its setting: The Land of Ooo looks harmless and magical, but it's really a post-apocalyptic wasteland. It's "Care Bears," and is therefore immensely enjoyable.

'The Muppet Show' (1976 - 1981)

The stars of "The Muppet Show" are basically the poster children (poster puppets?) for kids' entertainment that adults treasure even more. From the mind of Jim Henson, the prime-time variety show attracted A-list guest hosts like Gilda Radner, Liza Minnelli, Roger Moore, and Gene Kelly to "play the music" and "light the lights" with its motley crew of furry entertainers. Basically, "The Muppet Show" was dinner theater with puppets, and its pun-loving heckler duo Statler and Waldorf were a prophetic precursor to opinionated Internet comments sections.