Six seconds isn't a lot of time. If you were to read this sentence out loud, by the time you finished, six seconds would be up. But the brevity of Vine, the app that lets users make and share six-second video clips, has attracted 40 million registered users since its January 2013 launch.

When popular photo-sharing app Instagram added video to its capabilities this summer, time was part of its distinction. Instagram allows users to take 15 seconds of video; Vine limits you to six.

And something about that limit is appealing. I asked Vine co-founder Dom Hofmann if one day he had a revelation that six seconds was just right.

"One day we did wake up and say, six seconds," Hofmann joked. Well, one day after many days of experimentation.

He and the other co-founders tried various lengths — 10 seconds, nine, five. And five seconds wasn't long enough.

"It was actually too short," he says. Six seconds allowed for the aesthetic feel the creators wanted but preserved the quickness they wanted to promise users. The limit allowed the average person to easily share and make a video on his smartphone.

But once they settled on six seconds, something was still off.

"The next thing that we noticed was that the videos start quickly but they also end very quickly and that felt anti-climactic," said Hoffmann. "It didn't feel right."

That's when the founders added a loop. On the Vine app, videos play over and over, and that loop has allowed for creative artists to play with "infinite" video action, and babies on Vine to take their first steps ... over and over.

"You don't just skip a six-second video, so you watch it. And when you like it ... you appear to watch it three, four, five, six times in a row," said Pierre Laromiguiere, president of Armstrong, a marketing company that uses Vine.

While Instagram can boast of bigger numbers, Vine's brand identity has become synonymous with artists, comedians and filmmakers who use the constraints of time — and the looping — for creativity.

Of course, creativity often happens when artists are given restraints. Haiku has only three lines, and it's been around for centuries.

Actor Adam Goldberg says, "Six seconds is the new black." Goldberg's put up more than 169,000 followers on Vine.

"The combination of the motion and the sound just kind of lent itself to something kind of eerie," he says, making it the perfect creative outlet for him. "Then you ... combine that with a tortured manic mind and, well ... you've got yourself a good time."

There's also a genre of animated Vines. Katy Lipscomb, a 19-year-old art student living in Georgia, says Vine makes the process of animation easy. "You just tap the screen ... you've got a frame and then you can keep on going," Lipscomb says.

Among her animations is a six-second video of a unicorn. The horn grows out of its head and the body fills in with colors until it looks as if it will jump out of the screen.

But Vine's six-second limit has its detractors.

"I just think that as we continue to dwindle down our attention span, eventually it's going to be about zero point five seconds and that'll be praised as the next big thing," says the blogger Carlos, who writes for the sports and pop culture blog Grantland.

Below, one of our favorite Vines of all time:

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