Maybe it was in the water. Genetics probably had something to do with it. And then there's family.

For Tommy and Dick Smothers, the duo who form the Smothers Brothers comedy team, it was, well, maybe some luck that made them a success, Tommy Smothers says. And a lot of other factors. Like being really, really funny.

But if you were green with envy when you saw one Super Bowl champ, Peyton Manning, cheering on his younger brother Eli Manning to victory in Sunday's Super Bowl, stay green.

That's because "there certainly is not a formula" for raising super-siblings such as the Mannings or the next John and Joan Cusack or Malcolm and Steve Forbes, says Debbie Glasser, a clinical psychologist in Richmond, Va., and editor of www.NewsforParents.org.

Your odds of producing just one superstar are about as good as you winning "American Idol". Or less.

"A lot of people work hard, but only a few achieve that kind of notoriety and that kind of celebrity," Glasser says.

In fact, if you insist on trying to produce super-siblings by using the age-old parenting style of pressure and sibling comparison, you might find yourself with a pair like Jimmy Carter, the famous former president, and his infamous brother, Billy Carter, whose claim to fame had something to do with beer.

Pressure may appear to work in the short term, says Wendy Grolnick, professor of psychology at Clark University in Worcester, Mass., and author of "Pressured Parents, Stressed Out Kids". But, she says, "it will backfire.

"You may get your kids to do some things," she says. But, she says, "parents who are really pushy and controlling really do undermine kids' underlying motivation."

The best strategy, experts agree, is the old tried-and-true: nurturing. The majority of famous siblings "will say they were nurtured, not pushed," Glasser says.

And if you're thinking of telling one child to be as good as the other, scrap that idea.

"Growing up and hearing "Why can't you be like your big brother?' if anything, would turn a second sibling off of pursuing whatever it was that the older brother or sister was doing," Glasser says.

On the other hand, sibling rivalry just might be underrated.

Take the Smothers Brothers, who have pretty much based their 50-year comedy careers on sibling rivalry.

That wasn't the original idea, says Tommy Smothers in a phone interview from Kenwood, Calif. The two started as a singing act. But their onstage banter became the big draw.

"Dick and I have been at loggerheads from the time we could even talk," says Tommy, 71, who is 22 months older than Dick. "There's always been an argument." Early on in their careers, back in 1960 in Aspen, Colo., they even came to blows. "We were rolling in the snow in front of everyone."

Then one day on stage, Tommy turned to his brother during a show and said, "Mom always liked you best." It became their signature line.

The line - and whole act - resonates, Smothers says, "because it's a real discussion."

So who did Mom really like best?

When asked once, Tommy recalls, "She said, "I don't care for either one of them. I prefer my daughter, Sherry.' "

But in all seriousness, when asked what the secret formula is for producing famous siblings, Smothers says he doesn't know. In his case, "we came from a dysfunctional family." But he can give advice to siblings - especially those as different as he and his brother - who want to get along.

"Just get over the flaws, the differences," he says. "You're raised together, you're in competition in some ways you don't even know. Those are the things that you try to dismiss."

And, he says, "never carry an argument past a day."