Since the beginning of time mankind has posed the most difficult questions: "Is the Earth round or flat?" "Is there life after death?" "Why is junk food so tasty?"

But no question elicits quite the same response as: "Who lives in a pineapple under the sea?"

The answer, of course, is the star of Nickelodeon's enormously popular cartoon series "SpongeBob SquarePants," which enjoys nearly as many adult fans as it does children. After five years on television, the little sponge now hits the big screen with the aptly titled "The SpongeBob SquarePants Movie."

To pull off such a trick, the filmmakers must stretch story arcs that usually last 11-12 minutes to a full 90 minutes.

They have taken the easiest and most expected route, using Hollywood's accepted and limited screenplay format, including its three-act structure, its "call to adventure" and its "conflict of self." But within this structure, the filmmakers have as much post-modern fun as possible, including live-action sequences, music videos and some stunningly rendered set pieces.

Fortunately, the filmmakers do not pitch their movie to first-timers, sparing us 30 minutes introducing the characters. If you know SpongeBob and friends already, you're in. If you don't, well, this isn't Tolstoy.

As the movie opens in Bikini Bottom, SpongeBob's boss, the owner of the Krusty Krab restaurant, Mr. Krabs (voiced by Clancy Brown), prepares to promote a new manager for his second store. SpongeBob (voiced by Tom Kenny) hopes and expects it will be him, but the law of movies says that when a character expects to get a promotion, he will most certainly not get it.

This leads to SpongeBob's "crisis of conscience" -- he is not a grown man, just a little kid.

Meanwhile, regular SpongeBob villain and owner of rival restaurant the Chum Bucket, Plankton (voiced by Mr. Lawrence), launches another of his evil plans to pilfer the recipe for the successful "Krabby Patty," stealing King Neptune's crown and blaming the theft on Mr. Krabs.

SpongeBob and his stalwart pal, the starfish Patrick (voiced by Bill Fagerbakke) must venture into dangerous Shell City to retrieve the crown before Plankton can brainwash everyone in Bikini Bottom and "take over the world."

Special big-screen movie-star voices in the cast include Alec Baldwin as a hired killer, hot on the trail of SpongeBob and Patrick, Jeffrey Tambor (from TV's "Arrested Development") as King Neptune, and Scarlett Johansson as Neptune's levelheaded daughter Mindy. Best of all, David Hasselhoff plays himself as a real-world hero who gives our animated heroes a hand. After cameos in SpongeBob and John Waters' A Dirty Shame," could it be that Hasselhoff is headed for cult status?

"The SpongeBob SquarePants Movie" plays like "The Powerpuff Girls Movie" in that it's is very funny and awfully cool, but the laughs are superficial and the jokes fade from the memory quicker than witty e-mails. Indeed, some of the short cartoons are structurally more solid.

But SpongeBob is still SpongeBob, and his unflagging innocence and lack of irony make for a much-needed and much-appreciated hero.