Why, 20 years later, 'SpongeBob SquarePants' endures as one of the best kids' shows around

Twenty years later, that pineapple under the sea is still pretty fresh.

Hard as it is for the millennials who grew up watching to believe, "SpongeBob SquarePants" turns 20 on Wednesday. The beloved Nickelodeon show premiered May 1, 1999, and quickly became a pop-culture institution that shaped a generation of humor and remains relevant even into its original viewers' adulthood.

Although children's TV can be formative, few series have echoed the cultural footprint of "SpongeBob," which spawned a (short-lived) Broadway musical, two feature films (a third is due in 2020), comic books, video games, theme-park rides and even a wax figure at Madame Tussaud's.

Its anniversary will be celebrated this summer on Nick with "SpongeBob's Big Birthday Blowout," a new special mixing live action and animation and featuring the cartoon's voice cast playing human versions of SpongeBob, Patrick, Mr. Krabs, Sandy, Squidward and others.

But how, exactly, did an anthropomorphic sponge, a lazy starfish, a greedy fast-food restaurant-owning crab, a squirrel and a fastidious squid become cultural icons?

For folks in their late 20s and early thirties who were among the first "SpongeBob" viewers, the cartoon series earned a dedicated following by treating the kids who watched it as adults.

Nothing about "SpongeBob" is ever cutesy or condescending. Its characters are adults, if a little juvenile in their behavior. And "SpongeBob" makes adulthood seem fun and appealing. Its version is what kids themselves would dream up: Working at your favorite place, hanging out with your friends all the time and living in a cool pad with your pet. Even if it's underwater.

The series also gets in plenty of digs in at the worst part of growing up: Losing your sense of fun. Its main antagonist, Squidward, is a straight-laced rule follower who is always trying to bring SpongeBob's carefree attitude down.

SpongeBob's surrealist, odd sense of humor was far ahead of its time. Videos and screenshots from years-old episodes proliferate modern meme culture. It's harder to express your feelings better now than "SpongeBob" did then.

When you revisit famous episodes, it's easy to see the show's quiet brilliance (its running "my leg!" gag never gets old). The comedy ranges from fart jokes to dark capitalist satire.

"SpongeBob" finds nearly as much time for sophisticated parody as animated peers "The Simpsons" and "Family Guy" (which premiered four months before "SpongeBob"), even though it is aimed at a considerably younger audience. It boasts a brilliant superhero gag in recurring characters Mermaid Man and Barnacle Boy, and makes endless allusions, from Edgar Allen Poe to "Kill Bill." Writers – including series creator Steven Hillenburg, who died last year – were never afraid to be strange, loud and daring.

A few years ago, Gen Xers had their heyday of nostalgia when childhood cartoons such as "Transformers" and "G.I. Joe" were rebooted as big-budget films. The "SpongeBob" anniversary comes as millennial nostalgia is starting to take over (just look at all those 2000s fashion trends coming back). Thankfully, the little sponge has never gotten a gritty, PG-13 reboot, and probably never will.

The brilliance of "SpongeBob" is that it appeals to both kids and adults just the way it is: Peppy, a little grating and totally bizarre.

This article originally appeared on USA TODAY: Why, 20 years later, 'SpongeBob SquarePants' endures as one of the best kids' shows around