In the game of "sack tapping" or "ball tapping," as it is called, a boy will try to slap or flick the groin of another boy to inflict discomfort or pain.
The "game" has gotten out of hand, however, according to Dr. Scott Wheeler, a pediatric urologist in Minnesota.
In recent years he's seen an increasing number of young boys coming in with serious damage to the testicles because of a "tap" that hit too hard.
"For some reason, [this game] is popular," Wheeler says. "It's not just bullies, it's kids doing it to their friends. It's rare that bullies target the testicles, they just beat up people."
The trend isn't restricted to Minnesota. A search on YouTube reveals hundreds of videos of young boys, teens, and even members of the U.S. Navy, catching a friend (or enemy) unaware with a quick punch or slap to the genitals.
There's even a "Judge Judy" mock court case involving a 12-year-old "sack tapper" posted to the site.
In some cases, trauma sustained by sack tapping can be severe enough to rupture a testicle, an injury that can require surgery or even the amputation of the testicle, says Wheeler.
This fad is no joke, Wheeler says, "It's just ridiculous that boys are doing this."
The groin is one of the most sensitive parts of the body because it has such a rich supply of blood and nerves, says Dr. Aaron Katz, a urologist at Columbia University Medical Center in New York.
In the womb, the testicles form inside the body, so when they descend they take nerves from the abdomen with them.
"That's why when a guy gets hit in the balls, he feels it in his stomach as well," Katz says.
So why would boys do that to each other?
Dr. Charles Raison, an associate professor of psychiatry at Emory University in Atlanta, says the game might have a lot to do with asserting dominance. It does, after all, involve the "all-around central organ of dominance for man."
"Sack tapping resonates on a lot of different levels, some of which kids are probably not fully aware of," he says.
Raison compares sack tapping to the common game of "chicken," where boys take turns hitting each other to see who can take the most hits without calling off the game.
"Games like this are to see how tough you are," he says. "It's a way of establishing dominance and because it's hard to withstand being hit in the groin, it becomes a good measure of toughness."
"It's a pride thing, and by posting it to the Internet it's like proof. There's no doubt about your toughness; the whole world can see."
But where does dominance "play" leave off, and violence begin?
To suffer some of the injuries reported from this game, Katz says, the boys must be slapping with "quite a bit of force."
"The testicle is protected by a fibrous capsule. The force has to be fairly quick and really hard to break that capsule," he says. "You can fix a tear in the testicle, but if it's ruptured bad you have to remove the testicle because the tissue will die and cause infection."