For most people, working one job is plenty, or even more than plenty. But 7.3 million people -- about 5% of all American workers -- had multiple jobs in 2015, including 3.9 million holding down both full-time and part-time jobs, according to the Bureau of Labor Statistics.

What kinds of jobs are they working? When college student news and info site FlockU asked 14,000 students, "Which job is most appealing for supplemental income?" the big winner was "Uber driver," selected by 44%. Next up, Apple Genius was the choice of 30%, followed by Etsy Creator with 17% and Airbnb Host with 9%.

There are two clear themes to the sorts of second jobs Millennials are selecting, notes Josh Verne, Founder and CEO of King of Prussia, Penn.-based FlockU. "Overwhelmingly, it came through this on-demand economy," Verne said of the second-job preference hierarchy.

The second theme was name recognition. "It's the Ubers of the world, the Apples of the world," Verne says.

A common characteristic of all these positions is schedule flexibility. Uber specifically advertises that drivers have the ability to choose their own hours of work, Verne notes.

Flexible scheduling allows workers to fit secondary employment with primary job and daily living requirements, and it makes it easier to avoid conflicts with full-time work or schooling.

Charles Silberman is a teacher in Prince George's Country, Md., who two years ago took on a second job as a freelance writer. Silberman obtains his assignments through online service marketplace Fiverr.

"I decided to freelance for extra money to help with savings and paying down student loans," Silberman says. He says he's picked up an extra $10,000 and also enjoyed the opportunity to earn income doing something he loves.

"Freelancing also gives me ultimate flexibility," Silberman adds. "I can work when I want and where I want while controlling how much I work and on what I work on. For example, if I get sick and need to take a week away to rest up, I can. Likewise, if my full-time job is busy, I can decide to take on less freelancing work."

It's not all short workdays and high pay in the second-job world, however. Verne says students worry that second jobs will make it difficult to concentrate on grades and take time away from socializing. Silberman squeezes his freelancing during the school year into lunch breaks and before and after-school duties.

Silberman says his employer knows about his second job, and he's able to maintain separation between teaching and writing gigs. Much of his writing relates to health and fitness, which complements his work as a physical education teacher. Sometimes he is able to use written material he's created as a freelancer in his classroom work.

His employer hasn't complained about a conflict, but Silberman recognizes the potential. "The major risk is it can become all-consuming at times if I decide to take on a project I underestimate," he says. "In that case, it can get overwhelming and affect other areas of my life."

Between 2014 and 2015, the number of people holding second jobs ticked up slightly, according to the Bureau of Labor Statistics, while holding steady as a percentage of the labor force. Whether growing, shrinking or staying the same now, the idea of having a second job clearly holds considerable attraction as an income generator and, at least for people like Silberman, a source of satisfaction.

"As much as it could be a distraction -- and create sleep deprivation," Verne says, "it really is great for people and putting money in their pockets."