The person who now presides over a group that is trying to convince Philadelphia millennials to stay here says he came to Philly because of a napkin.

Nick Marzano and his girlfriend at the time, six years ago, were living in Steamboat Springs, Colo., a ski-bum’s paradise. They wanted a change, and each of them wrote three options on a napkin for where they could start a new life. Philadelphia was the lone choice they shared. So Philadelphia it was.

“I love where I’m at right now,” says Marzano, president of Young Involved Philly and director of digital learning at Society of Hospital Medicine.

His story is surprisingly unsurprising — nobody really knows exactly why Philadelphia has become a magnet for millennials. It doesn’t rank high for startups, tech, bikeability or many other measures favored by millennials. Its job growth rate is about half the national average, and only this year did the job number reach pre-recession levels and the unemployment rate drop below 9 percent.

But from 2006-2012, its population of 25-34 year olds increased by 100,000, a 6.1 percent growth rate. No U.S. city among the top 30 in population had seen as high of a rate. So trendy is Philadelphia that Forbes hosted its “Under 30 Summit” here last month. Starting today through next week is “The State of Young Philly,” an event (now in its fifth year) hosted by Marzano’s Young Involved Philly. Philadelphia seems to be all about millennials all the time.

Of course, very little good news comes without a catch. The buzzkill for Philadelphia is that its millennial-backed resurgence could be ephemeral. A Pew study from January revealed that 50 percent of millennials planned to leave Philadelphia in five to 10 years and only 19 percent thought they would definitely stay.

No one can explain exactly why Philadelphia has become a hub for millennials. But keeping them here will likely take planning and coordination.

Luke Butler is one person trying to keep young people in Philadelphia. He’s the chief of staff to the deputy mayor for economic development. He says the city has always wanted to attract and keep young people but began ramping up its efforts a few years ago when the city’s overall population increased in a 10-year period for the first time since 1950.

“I think the 2010 Census was really kind of a galvanizing moment for the city,” Butler says.

Millennials’ biggest problems with Philadelphia right now are job opportunities, education and public safety, according to the Pew study. Pew found that most of the millennials it surveyed have come to Philadelphia for nebulous quality of life reasons – bars, restaurants, culture, affordability. Those attributes are attractive when you’re single but – aside from affordability – not generally priorities for older people with families.

Though the mayor’s office has no specific team working on developing a city that will keep millennials around, Butler says multiple city departments work amongst themselves, as well as with private organizations, to work toward this goal. They’ve also looked toward other cities for examples. Some of the progress he noted includes the StartUp PHL initiative and the city’s bicycle lanes and new bicycle advocacy committee. He admitted education reform would provide a formidable challenge, separate from most everything else.

Larry Eichel, director of Pew Charitable Trusts’ Philadelphia program, says the good news for Butler and others planning for the future of the city is that millennials share much of the same interests with older residents. Their list of priorities is heavy on public safety, career concerns and schools, Eichel says. For millennials, it’s career concerns, schools, public safety.

“If you take care of the fundamentals you address a lot of the concerns millennials have,” Eichel says.

On the negative side, Eichel says, it’s not clear what Philadelphia can really do. No research or data has been collected for proven methods on how to keep millennials in cities; they’re approaching the age where many will start families. The best the city can do is continue to grow jobs, work on the seemingly never-ending schools problem and hope that’ll steer at least a few young people away from the ‘burbs, or New York, Washington or San Francisco.

If there’s any silver lining from the Pew data, it’s that Philadelphia can at least commiserate. Nearly every other city shares the concern of possible millennial exodus. Census data illustrates that for as popular as city life is getting for the younger crowd, cities are still losing more people than gaining. From 2005-2010, 11 million people moved into cities and and 15.4 million moved out. And the suburbs, the eventual destination for young people with means, saw 17.9 million move in and only 9.2 million move out.

Cities throughout the United States are searching for solutions to keep millennials, who nationally seek many of the same things as Philadelphia millennials: good schools, jobs and walkable/bikeable urban areas.

The stakes are high. There are an estimated 86 million millennials in America, a group more populous than even the baby-boomers, who number 77 million. The sheer quantity of millennials means Philadelphia needs to keep many of them if it wants its population to keep growing. The generation that follows the millennials – even it proves to favor urban areas at a young age, just like millennials – won’t bring as large an influx.

“People who are going to be young adults 10 years from now, they’re born already,” says Eichel. “And there aren’t as many of them as millennials.”

Another thing in Philadelphia’s favor is that its millennials haven’t just been moving to Philadelphia; they’ve been moving smack-dab in the middle of Center City. According to a study by Joe Cortright in City Observatory, Philadelphia, as of 2010, had just over 50,000 college-educated 25-34 year olds living within three miles of the city’s main business district, a 78 percent increase from 2000. Only seven other cities out of the 51 biggest in the U.S. had a larger increase.

The sizable population of college graduates in a small area is conducive for fostering relationships and a sense of community — two tenets Marzano says Young Involved Philly considers necessary for keeping millennials around.

Marzano isn’t married and doesn’t have kids yet. He says he doesn’t begrudge anyone for leaving if they are starting families. Though his arrival to Philadelphia was contingent on the random luck of a napkin, he plans on staying for the future.

“I’m looking into buying a house,” he says.