Among the (many, many) things you probably do not know about juggling is the fact that it is, at times, a physically grueling act. It's something I certainly failed to appreciate before meeting Alex Barron. We recently met at a squash court in Burbank, California so I could watch him practice his craft. There, the enclosed space shelters him from trajectory-wrecking winds, the high ceilings afford him ample space, and the white walls provide a uniform backdrop, ideal for passively tracking the movement of arcing objects—at times, a dozen or more of them.

At 23 years old, Barron is the world's best juggler of 10, 11, 12, 13, and 14 balls. His physique is statuesque: 6 foot 3, big shoulders, little joints, cover-model muscles. When he's not setting records in numbers juggling (the practice of juggling high numbers of objects), he surfs and rock climbs, but neither activity exhausts him quite like lobbing pellet-filled pouches high into the air. "The physicality of it I find quite hard," he says between heavy breaths, a puddle of sweat collecting at his feet. "Practicing high numbers two days in a row is just really rough on my body."

In October 2011, Barron became the first person to flash 13 balls on video (in a flash, the juggler throws N balls and executes N catches), a feat that revived longstanding debates over the absolute limit of numbers juggling. Practitioners often put the ceiling at 14, a number whose experimental origins can be traced to a paper published in 1997 by stuntman Jack Kalvan. A mechanical engineer and lifelong juggler, Kalvan devised a simple mechanism for measuring the speed and acceleration of jugglers' hands. From the data he collected, he concluded that someone would eventually juggle 13 balls—and while he never went so far as to claim an upper limit, he did write that "flashing 15 doesn't seem too unlikely."

The paper was controversial in juggling circles. Peter Bone, an accomplished numbers juggler and occasional competitor of Barron's, took issue with Kalvan's methods. "There are so many variables that aren't taken into account by simply measuring hand acceleration," he wrote on jugglingedge.com, in a thread about the empirical and anecdotal bases for a 14-ball threshold. "I think the 14 ball claim just comes from the experience of numbers jugglers in general. For example, Ben Beever [another premier numbers juggler] and I ... think that a 14 ball flash is possible, but not 15."

Bone wrote those words in December of 2012. Not six months later, Barron broke his own record by throwing and catching 13 balls 15 times. "I am working on [a 14 ball flash]," he wrote in the notes of the YouTube video he posted as proof, "but it may be a while."

It took him nearly four years. In April 2017, Barron became the first person to flash 14 balls on video. To appreciate everything that's going on, you'll want to watch this footage several times; it happens that fast:

An instant before liftoff, Barron dips into a shallow squat, loading his legs with the extra energy he'll need to loft the first ball into the air. Though the bean bags weigh just 70 grams apiece, it takes him as much effort to throw each one as it would to hurl the entire contents of his hand. The first toss, then, feels, to Barron's right hand, equivalent to lobbing 1.25 pounds high into the air. The second and third tosses: a hair over a pound each. The fourth and fifth: just under a pound apiece. Only on the 13th and 14th throws does propelling the bag feel like flinging a mere 2.5-ounce sack.

Except flashing this many balls takes more precision than a word like "flinging" suggests; it demands something more like perfection, 14 times in a row. In juggling circles, a consistent, five-ball pattern is the sign of devoted practitioner, the culmination of tens if not hundreds of hours of deliberate practice. Seven balls juggled commands respect; eight or more a level of admiration approaching reverence. As one of the commenters on Barron's 14-ball video put it : "Each number is exponentially more difficult. I got up to qualifying 9 and flashing 10 and worked for 10 more years to try to get it solid and never could. So to see you juggle 11 and flash 14 is phenomenal! A true feat for the human species! Congrats!" (Commenters in the YouTube juggling community are an uncharacteristically earnest and humane bunch.)

The speed, the coordination, the sheer physicality of it—they're almost impossible to fathom. Almost.

In the two decades since he first wrote on the subject, Kalvan has continued to study and characterize the limits of numbers juggling. To his previous analyses, he has added exhaustive, juggling-specific examinations of things like hand range (i.e. the spectrum of positions from which a juggler can conceivably throw and catch objects), collision avoidance, reaction time, and effort. "All the good stuff is included in this new work, but better," Kalvan says. He plans to publish a book-length work on the subject later this year.

The upshot of his analyses: Bone was right. Hand speed is just one of the limiting factors when it comes to numbers juggling—and a small one, at that. Far more crucial, Kalvan says, is accuracy.

By building on the juggling theory of Claude Shannon (yes, that Claude Shannon—the father of information theory was also an avid juggler), Kalvan's studies detail the ways that juggling more balls requires a simultaneous increase in the height, frequency, and precision of one's throws. And as you might expect, getting a ball to land where you want becomes increasingly difficult, the higher and more quickly you throw it.

This past week, Kalvan invited me and Barron to his Los Angeles home to meet up, talk about his findings, and run us through an experiment. Behind his house, in a high-ceilinged acrobatic practice space, Kalvan had Barron and I take a crack at his hand-speed test. With an accelerometer strapped to our wrists, Barron and I both spent a few seconds air-juggling as fast as we could (picture a mime juggling on fast-forward). Then we each spent some time juggling real balls. By comparing our actual juggling speeds to those of our mime routines, Kalvan was able to produce a theoretical limit for us both. Apparently, my hands move fast enough to flash 14 balls. As for Barron, Kalvan calculates his hands are quick enough to handle 25—faster than he's ever measured.

Barron might be better equipped than anyone to break his own record. But doing so could require many more years of training—and no small amount of luck. Kalvan hypothesizes the vast majority of people who have thrown and caught more that nine balls have avoided midair collisions mostly by chance. Barron might be an exception, but even he, Kalvan suspects, relies on skill and serendipity alike when juggling 13 balls or more.

In that light, the key to flashing 15 balls could be hidden in the law of large numbers. With enough attempts, someone like Barron might just be able to make his own luck.

More Great WIRED Stories

• CGI "influencers" like Lil Miquela are about to flood your feed

• Insect-borne diseases have tripled. Here’s why.

• Oculus Go review: Cordless VR is here and it’s pretty dope

• Star Wars is becoming a religion, and May 4 is its spring festival

• That time when John Doerr brought a "gift" to Google’s founders