Richie Hartzel's friend meant no harm during the sleepover last January when he started rat-tailing a towel at the 13-year-old's back. But in that critical fraction of a second when what-ifs become what-nows, Richie turned to look over his shoulder. The tip of the towel snapped into his left eye, detonating in the cornea, the eye's outer layer, and ripping the iris that controls the amount of light that enters the eye.

Richie's parents rushed over to drive him to the hospital and found that their son's entire eye had gone horrifyingly black.

The emergency room doctors said there was not much they could do, other than give Richie a patch and wait to see whether the eye would heal.

"They told us it could take six months," said Richie's mother, Suzanne. By the end of the school year, he seemed fine, although the injured eye remained blurry and he saw halos around lights.

"We kept expecting it to get better," she said.

In July, when Richie, who plays nose guard for the Owens J. Roberts High School football team in Pottstown, went for his ninth-grade sports physical, the doctor found a cataract.

This clouding of the lens - a buildup of protein that blocks light - usually develops as people age. About half of Americans over 65 have them.

When children and teenagers such as Richie get cataracts, the cause is usually trauma.

Worldwide, nearly 250,000 children under 15 suffer eye injuries each year - often with catastrophic consequences.

"Anything that can go into an eye, will go into an eye," said David Hunter, clinical spokesman for the American Academy of Ophthalmology and ophthalmologist-in-chief at Boston Children's Hospital. "We can't just put our kids in motorcycle helmets with polycarbonate face shields."

Nearly 41 percent of eye injuries in children between age 10 and 14 are inflicted by sports equipment such as BB guns, arrows, bats and balls, according to the academy's statistics.

And although catching the business end of a snapping towel is not high on the list, household products provide an impressive arsenal of objects that burn, pierce, bash, irritate, poke, and slash kids' eyes.

A partial list, drawn from emergency room records, includes bedsheets, beer bottles, belts, books, brooms, buckets, chopsticks, Christmas tree lights, clotheslines, crayons, erasers, flashlights, keys, magazines, mailboxes, paper clips, pillows, rugs, shower caps, staplers, teapots, toothpicks, and zippers.

"As an ophthalmologist who is the father of two boys," Hunter said, "I never allowed them to pick up sticks and play with them."

Setting limits like this is hard. "I played with sticks when I was a little boy," he said. "But I see what happens when things go wrong."

At this time of year, parents need to use common sense when choosing gifts.

"Especially with projectile toys," Hunter said. "If it seems like it probably isn't going to be safe, it probably isn't going to be safe."

One of the most popular toys this year, he noted, is a crossbow that can shoot more than 100 feet.

"I know it feels bad to not buy the most popular toy for your kid. We just have to find a balance between what's sensible and what's obviously dangerous."

Paintball injuries, he said, are particularly devastating. The balls are "just the right size to cause the maximum damage to the eye." Although everyone wears protective equipment during the games, he said, "Accidents happen after, when they're horsing around."

Compared with many young patients, Richie was lucky, said Steven Siepser, an ophthalmologist who operates at Wills Eye Hospital and an outpatient surgery center in Exton.

"This isn't the worst I've seen," Siepser said one morning in mid-December as he prepared to remove Richie's cataract. Siepser, 68, said that every few months, he sees children with eye injuries.

"It's always something like, 'My brother threw a pencil at me,' or 'I was hit with a Frisbee.' "

Despite his damaged eye, Richie had been functioning well, his mother said. On the first day of hunting season this year, he shot two deer in the family's backyard in Pottstown, where they raise chickens and pigs.

"He's an expert marksman," she said. Without his realizing, though, Richie's right eye was working hard to compensate for the loss of sight in the left.

After receiving antibiotic eyedrops and a medicine to dilate his pupil, Richie, who is now 14, was rolled into the operating room and given general anesthesia. Within minutes, Siepser, peering through a microscope, had cut into the gelatinous part of the boy's eye and delicately extracted the diamond-shaped cataract. He then slipped an artificial lens into place and, with a few tiny sutures, completed the job.

Half an hour later in the recovery room, Richie blinked his eyes open to find his mother, worried but smiling at his bedside.

"How do you feel?" she asked.

Her son turned to Siepser and groggily asked, "You fixed both eyes?"

"Just the one," Siepser said, chuckling.

After reviewing the postoperative care with Richie's mother, the doctor then discussed his long-term prognosis. Although the new lens would dramatically improve the boy's sight, he will face a lifelong risk of other problems, including glaucoma and a detached retina.

His best chance of protecting his eyesight, Siepser said, will be to avoid more trauma. And that, he said, means finding a less head-thunking sport than football.

"No," Richie said.

The following week, Richie and his mother, bearing a bounty of Christmas cookies, returned to see Siepser for a checkup. With his good eye, the boy could read down to the tiniest print on the chart. With the other, he could make out only the relatively large letters.

That will improve as the eye continues to recover, Siepser said. But he had deliberately adjusted Richie's sight to favor reading over long-distance, he said.

"His reading vision is perfect," he said. "You're coming along great."

Richie thanked the surgeon and, shaking his hand, gave him the news.

That football career?

"I'm probably going to stop playing."

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