At a festival in Germany, when six women walked by wearing specially-created "SpongeBob SquarePants" shirts, Stephen Hillenburg's wife nudged him: Why don't you tell them who you are?
Hillenburg, who created SpongeBob and all his undersea friends at the Krusty Krab, declined.
"I thought that it was so surreal, they probably wouldn't believe me," he recalled Monday.
Believe this: television's favorite animated sponge is now 10 years old, and Nickelodeon celebrates this weekend with a blowout bash. Eleven new episodes and countdowns of fan favorites will air. A documentary on the series debuts Tuesday on sister station VH1.
"SpongeBob SquarePants" has been TV's most popular animated show for children aged 2 to 11 for seven years now, and a not-so-secret factor to its appeal is that many parents — and even people without kids — love it, too. It's a cash cow that has generated $8 billion in merchandising revenue for Nickelodeon.
The show is seen in 25 different languages and counts two world leaders, President Barack Obama and British Prime Minister Gordon Brown, who say they watch with their children.
Hillenburg is an artist and also a scientist who taught, and the inspiration for many of his show's characters came from a comic he wrote, "The Intertidal Zone," designed to teach his students at the Ocean Institute about tidal pool characters.
Through 125 episodes and guest shots by the likes of David Bowie, LeBron James, Whoopi Goldberg and Ray Liotta, the series has evolved and explored other characters to avoid repeating itself. But the essence of the show remains SpongeBob and his sweet innocence, Hillenburg said.
"I never really imagined a show about a sponge going past our first season," he said. "I thought maybe we'd have a cult following, and we'd be gone after one season. I'm in disbelief that we're here talking."
Cyma Zarghami, president of Nickelodeon and MTVN Kids and Family Group, was one of four executives in the room when "SpongeBob SquarePants" was screened for the first time. Their immediate reaction was to see it again, both because they liked it and it was unlike anything they'd ever seen before, she said.
The show's optimism, as it first appeared a few months after President Clinton's impeachment trial, was key to its rise, she said.
"You can put all of the right ingredients into the pot when making a show like this, but there is some kind of magic ingredient that you can't account for or predict, and that sends it into the stratosphere," she said.
The first episode aired on July 19, 1999. A month later, the show's first celebrity guest voices appeared — Ernest Borgnine andTim Conway.
This weekend's "SpongeBob SquarePants" marathon will include, besides the new shows, separate top-10 countdowns of favorite episodes as selected by fans and celebrities.
Nick currently has enough episodes on order to bring the total to 152 over the next year. The success of "The Simpsons" notwithstanding, that's an unusually large amount of shows in a television genre where they usually come and go quickly.
Zarghami said she sees no end in sight.
"As long as we are able to sustain the integrity of the characters and the quality of the program, I think we should continue," she said. "I don't think there will be a point of diminishing returns."
Since the "SpongeBob" movie in 2004, Hillenburg has stepped back into an executive producer's role. He no longer writes or runs the show on a day-to-day basis, but reviews each episode and delivers suggestions.
He's working on two other TV projects that he doesn't want to discuss and does a fair amount of painting.
"I figure when I'm pretty old I can still paint," he said. "I don't know about running shows."
Related Content from AccessHollywood.com:
VIEW THE PHOTOS: Barack Obama
PLAY IT NOW: Kids' Choice Awards 2009 Promo: Justin Timberlake & Dwayne Johnson
PLAY IT NOW: The Jonas Brothers Talk Performing At Nickelodeon's 2009 Kids' Choice Awards