Are you ready, kids?

(I can’t hear you!)

“If in nautical nonsense you’d like to plunge, then drop on the deck and mop like a sponge.”

For adults not yet initiated into the cult of SpongeBob, prepare to be swabbed by the porous yellow innocent with googly eyes, bucktooth grin, and cardboard-box shorts. According to the keepers of the stats, “SpongeBob SquarePants” is the most-watched children’s television show. Roughly a third of its audience is older than 18.

The star of the Nickelodeon series, and now of his own cheerily delirious movie, “lives in a pineapple under the sea,” or so the lyric of his theme song has it. But in fact SpongeBob amphibiously straddles the surreal and the real worlds as did Groucho, Pee-wee and Tinky-Winky before him. (Admittedly, SpongeBob is a little on the racy side, but understand that the bare butts we see belong to a sponge and a starfish.)

“The SpongeBob SquarePants Movie” gets a charge from the way its hero matter-of-factly breaches the membrane between second and third dimensions. Consider that the spongeboy is equally capable of driving a fuel-injected crab sandwich to Shell City, the Oz of this story, and of boarding the back of “Baywatch” star David Hasselhoff, who conveys the sponge home to his undersea community of Bikini Bottom.

The trippy creation of onetime marine biologist Stephen Hillenburg, SpongeBob is a cockeyed optimist toiling at the bottom of the fast-food chain. The plucky sponge (voice of Tom Kenny) flips patties at a joint called the Krusty Krab. Because he’s always being named Employee of the Month, SpongeBob naturally assumes he’ll be promoted to manager.

When his coworker Squidward, the undersea Eeyore, gets the job instead, SpongeBob is embarrassed, but he’s also free to see the world outside the Krab. With his sidekick Patrick, a starfish (Bill Fagerbakke), the Sponge embarks upon a Frodo-style odyssey to retrieve the stolen crown of King Neptune (Jeffrey Tambor) and experience a range of underwater life.

The lesson of SpongeBob, as most kids know and this grumpy parent has taken years to concede, is to maintain his goofy smile and optimism in the face of disappointment, disillusion and disaster. Nothing stops the Sponge, who irrepressibly bobs from one encounter to the next. (My biology teacher taught us that sponges lack nervous systems, which might account for SpongeBob’s temperament.)

Like filmmakers ranging from Andrew Stanton (“Finding Nemo”) to Wes Anderson (the forthcoming “The Life Aquatic With Steve Zissou”), Hillenburg is smitten by current events of the oceanic kind. Thirty years ago his kaleidoscope of beachcomber imagery (pineapples! tiki sculpture! batik prints!) set to ukulele music and waverider jabberwocky would have invited jokes about pothead lifestyle or parrothead excess. Today this surfer zen has an almost wholesome quality, as if the scent of tropical fruits as the sun melts on the horizon and waves lap at the shore is a natural psychedelic.

A recurring motif of the SpongeBob cartoon is the dominant theme here: namely, that ignorance is bliss. It’s not that Hillenburg believes that stupid is better than smart. It’s that, if you don’t know you can’t do something, then you’re more open to adventure.

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