In its year in review for 2017, GoFundMe listed Kate McClure's campaign for Johnny Bobbitt Jr., the man without a home who gave her his last $20 for gas, as one of its unforgettable "Strangers Helping Strangers" of the year.

But what made it unforgettable — and what made it go viral — may be as illusive as the $400,000 the campaign raised for Bobbitt that his lawyer says is now missing.

Ethan Mollick, a management and entrepreneurship professor at the University of Pennsylvania's Wharton School who has extensively studied crowdfunding, said the honest answer to what makes a campaign go viral is "We don't know."

"If we knew what made things go viral, whichever academic or business figures that out will do very well for themselves," he said.

But don't certain subjects — like a man experiencing homelessness giving his last $20 to a woman in need — tug at the heartstrings?

Johnny Bobbitt Jr. (left), Kate McClure, and McClure’s boyfriend, Mark D’Amico, at a Citgo station in Philadelphia during happier times in November.
(Elizabeth Robertson/Philadelphia Inquirer
Johnny Bobbitt Jr. (left), Kate McClure, and McClure’s boyfriend, Mark D’Amico, at a Citgo station in Philadelphia during happier times in November.

"If you're on GoFundMe they all tug at the heartstrings. It's full of sad stories," Mollick said. "National tragedy is one thing, but to nationalize an individual story is rare."

Most people who create crowdfunding campaigns see their greatest number of donations from family and friends and then from members of their online community, Mollick said.

"It's rare to have strangers commit cash," he said.

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But it happens. Perhaps it was a slow news day, which led Good Morning America to pick up the story after local news outlets reported on it, drawing national attention to the campaign. Perhaps just the right social-media influencers read the GoFundMe page and shared it with their followers.

Whatever the reason, going viral is less of a well-defined science and more of a strange and incomprehensible art.

What's likely to make a crowdfunding campaign succeed at a more modest level, however, is more tangible. Mollick said spending time on a good pitch, having a good video and accuracy have all proven to be important. A single spelling error in a campaign can decrease funding by 13 percent, he said.

In a field crowded with campaigns for medical emergencies and scholarship funds, a distinctive campaign stands out, said Sunil Wattal, associate professor of management information systems at Temple University's Fox School of Business, where he studies crowdfunding.

"It's not about the need, it's about how different your need is from the needs of others out there," Wattal said. "This was a story that was very, very unique. Nothing like this had been funded through GoFundMe before."

Tips for a successful campaign GoFundMe lists on its website include posting frequent updates, sharing the link to the campaign on Facebook, and using a bright image or video that includes the people involved.

According to GoFundMe, the most successful campaign on its site in 2017 was one that raised more than $11.7 million for victims of the October mass shooting in Las Vegas. The second most successful GoFundMe campaign last year raised more than $2.7 million for famine relief in Somalia.

Several campaigns that raised money for individuals made it into last year's top-grossing GoFundMe pages, too, like one that raised $1.8 million for an experimental treatment for a sick British baby and one that raised more than $800,000 for a California rugby player paralyzed by a sports injury.

Given that about 70 percent of crowdfunding projects fail, the one McClure ran for Bobbitt was unusually successful, Wattal said.

"If this happens again, though, I'm sure that it wouldn't raise as much as this campaign," Wattal said, because the cause would no longer be unique.