Most parents tell their children to stop fighting -- they scold and punish children who hit, kick or roughhouse.
But these days some moms and dads are doing just the opposite. They're encouraging their kids to fight, taking a cue from TV shows like "The Ultimate Fighting Championship," or "UFC," where moves like the "Ground Pound" and the "Cobra Strangle" have millions of fans hooked, including kids.
In "Ultimate Fighting," competitors box, wrestle, kick and do almost anything they can to knock out their opponent or make them submit. Now gyms around the country are teaching mixed martial arts, or MMA, the sport featured on the show, to kids as young as five.
Mixed martial arts fascinates people -- mostly male people, frankly.
Ultimate Fighting Championship, the sport's biggest promoter, caters to guys who always wondered whether karate is more effective than judo, or who would win in a battle between a wrestler and a boxer. MMA answers those questions, and the success of UFC has spawned a slew of spin-offs and feature films.
And now kids want to do it. Some tape themselves fighting and post the video on YouTube, and sometimes adults even construct cages so kids can fight like the pros.
Other children fight in gyms, on wrestling mats, and like the pros sometimes they get hurt. But unlike the pros, these young fighters sometimes cry and need to be comforted by their parents, as happened at a recent tournament in Santa Ana, Calif.
Some say MMA is a terrible thing for kids to be doing. Presidential candidate Sen. John McCain called the adult version "human cockfighting" and wrote a letter to the governors of every state asking them to ban it. Many states have.
Mayor Robert Correia of Fall River, Mass., was horrified to learn that in his town, not far from his office, one very successful ultimate fighting academy called Gillett's Gym teaches MMA to young boys and girls. The mayor says the sport sends the wrong message to kids and wants the gym shut down.
"To allow this to be taught to our children and for adults to stand by and cheer this on?" he said. "It's telling them 'look, the best thing to do is hurt someone.'"
But do the parents of the kids at Gillett's think the sport will teach kids to settle disputes with fists instead of words?
"No, my kids have never left here and used it outside of this gym," one parent said. "Never."
In fact, parents often get in the ring themselves, and some said that it's a good way to interact with their kids and that it gets aggressive impulses out in a safe and fun way. There are rules against things like punching in the neck and twisting fingers.
"He likes it as a sport and he likes it to teach him right from wrong," said Mindy Poirier, of her son's MMA experience.
Kim Velozo's 13 -year-old daughter Nicole fights, and Velozo said it's about self-defense.
"If somebody jumps her or something like that, she'll know how to defend herself against whatever," she said. "She loves it. She loves it."
Many of the moms said the training even helped their kids outside the gym, with discipline and following directions.
All the moms "20/20" talked to said mixed martial arts has been good for their kids. Correia doesn't think those parents know how unsafe it is.
"I've heard from doctors who are very concerned about this going on," he said.