For many newspaper readers, flipping back to what used to be called the funny pages is bittersweet. It's dependable amusement, yes, with Funky and Garfield and Beetle, but it's also a daily reminder that someone's missing.

Scanning the strips is like gazing out the window to the old maple next door . . . and its empty swing, swaying in the breeze.

It's been 15 years since Calvin and his tiger buddy Hobbes pulled up and rather suddenly left the comics pages. At the time, in 1995, the strip was at the height of its popularity, running in a staggering 2,400-plus newspapers and reaching an audience in the hundreds of millions.

Then, with a short note citing shifting interests and "the constraints of daily deadlines and small panels," creator and Clevelander Bill Watterson retired his masterpiece. The artist, whose reclusiveness -- and genius -- are often compared to the late J.D. Salinger, was still in his 30s.

Fans, who had enjoyed 10 years and more than 3,100 installments, were left without a daily face-to-face with the spiky-haired 6-year-old who had become a part of American culture. They couldn't even hug a stuffed Hobbes or watch an animated special or put on a Calvin T-shirt (an authorized one, at least) because of Watterson's stubborn refusal to license away his characters.

It was cold turkey -- and many fans continue to feel withdrawal.

"Still, people come up to me, and they grieve the loss of 'Calvin and Hobbes.' It's genuine," says Lucy Caswell, curator of the Billy Ireland Cartoon Library & Museum at Ohio State University, the renowned research facility that houses almost half a million original works of cartoon art, including all but about a hundred of Watterson's original strips.

The reason they mourn, she says, is that they had made friends with Calvin and his tiger. When he left, there was true emptiness.

Unlike other popular art of the era -- the films of Kevin Costner, perhaps, or the music of Bryan Adams -- "Calvin and Hobbes" has not been time-stamped and filed away. It has endured, even thrived.

Reruns of the strip, no longer available to newspapers in North America, still appear in more than 50 countries around the world (Miss Wormwood sends Calvin to the corner in Chinese, Vietnamese and Arabic). With little or no marketing, the "Calvin and Hobbes" compilations, now numbering 18 books, still sell half a million copies a year, according to Universal Press Syndicate. Total sales are nearing 45 million.

(Released in 2005, "The Complete Calvin and Hobbes," a three-volume collection of every C&H strip, has sold more than 500,000 copies. Perhaps not all that impressive -- until you realize the set weighs 23 pounds and retails for $150.)

Bootlegged Calvin merchandise is still ubiquitous, from fraternity T-shirts to the back windows of pickups. Fan Web sites abound. Search "Calvin and Hobbes" on YouTube, and you'll find dozens of attempts to animate the characters, some impressive, some embarrassing.

This summer, the

will release a "Calvin and Hobbes" stamp. And in front yards every winter, kids re-create Calvin's snow-sculpture masterpieces: frozen figures in various scenes of comedy, tragedy or horror.

Just recently, fan and author Nevin Martell penned a book, "Looking for Calvin and Hobbes" (Continuum Publishing, $24.95), which chronicles not only the story behind the strip but Martell's personal quest to track down the famously reclusive Watterson, who still makes his home in Northeast Ohio.

Why does "Calvin and Hobbes" have such staying power?

"A long list of things in my view," says Caswell. "Charming characters, it's beautifully drawn, engaging things happen, unexpected things happen.

"It was written with a respect for the readers," she says. "The assumption was the readers were smart enough to get it. I think people appreciated that. They responded to that."

It's timeless, too, she says, tapping into the enduring theme of a child and his fantasy world.

"If there is a 'Huckleberry Finn' of comic strips, this is it," says Universal Press Syndicate President Lee Salem, in an e-mail interview. Salem was Watterson's editor and the person who first opened his submission package to Universal a quarter of a century ago. He still remembers his first reaction.

"It was a breath of fresh air."

Yet he wondered whether the strip would speak to children.

"I thought it was perhaps too 'adult,' too literate," says Salem. "When my then-8-year-old son remarked, 'This is the Doonesbury for kids!' I suspected we had something unusual on our hands."

The strip debuted Nov. 18, 1985, in 35 papers. The Plain Dealer began running it the next March. By 1987, it was appearing in more than 300 newspapers, making it the fastest-growing comic strip of the decade -- and making Watterson, once a struggling artist, an almost instant celebrity.

In an interview with The Plain Dealer in 1987, Watterson said he was "shell shocked" by the attention. "The celebrity aspect of the job has taken me aback," he said, "and I really can't stand it."

"He genuinely appreciates his fan support and interest," says Salem, "but he remains perplexed that people would want to know about him or his life."

Because fans looked at Calvin as a friend, many felt the same connection with his creator, says Caswell.

"We thought we knew Bill Watterson, too, or ought to be able to know him," Caswell says. "It really is a problem."

Because fans thought they knew him, Watterson's decision to retire Calvin at the top of his game was puzzling and, yes, painful.

Caswell urges fans of the strip to focus on what Watterson gave them -- she calls it a "gift" -- not what they think he took away.

"I think we have to respect his choices," she says. "It seems to me that any creative person has the right to decide if they are or they are not going to make their art. . . . We on the outside can't judge whether or not it was the right thing for him.

"That ultimately has to be his choice."