It sounded like a twig snapping in half - sharp, pointy, brittle.
British mom Shona Keetley heard it when she and her 18-month-old son, George, glided down a big, colorful children's slide together, George sitting between her legs - like American pediatricians told parents last year to stop doing.
George was too little, she decided, to slide down alone, so she went with him to protect his tiny body with hers.
Halfway down the slide, though, his shoe got stuck on the slide and his right knee folded up under him.
His mom heard his leg break.
Now she wants other parents to hear that cringe-inducing sound, to see his tears, to see the flash of shock on her face, to heed her warning.
British parenting website Channel Mum posted the video to YouTube on Thursday as a public service announcement with a warning that it "features some distressing footage which may not be suitable for all viewers."
“I just didn’t think it would end up in a broken leg," Keetley wrote on Channel Mum's Facebook page. "We were actually on a family day out and the play area was in the building before the exit.
"My son was screaming that he wanted to go on the slide so I said to my partner that I would take him down once and then we would leave. That’s why I didn’t bother to take our shoes off – because it was supposed to be quick."
Her tragic slide was the second one to make headlines over the last few days.
American mom Heather Clare has a slide-related PSA of her own. In fact, she posts it every year on Facebook and this year it caught viral attention.
In September 2015, at a park in Huntington, New York, Clare put her daughter, Meadow, then 12-months-old, on her lap to go down a slide, just like Keetley did with her son.
On the way down, Meadow's foot got caught between her mommy's leg and the edge of the slide and bent backwards.
Meadow's little leg broke in multiple places - just three days after she took her first steps.
"This picture is the moment her leg was breaking. She’s still smiling. ... because it was happening at this exact moment," Clare wrote with this year's post, which went up on June 24.
Clare retold the story this year to the "Today" show, People magazine and other media outlets.
“When we got down to the bottom of the slide, she was crying bad,” Clare told People. “It was terrible, especially feeling like I put her in that position. It was completely avoidable.
"When we went to the ER, the super empathetic doctor lectured me on how common this injury is. I had no idea. I thought everyone took their kids down the slide."
Thousands of people do.
Last September the American Academy of Pediatrics released a report that said nearly 353,000 children younger than 6 were injured on playground slides in the United States from 2002 to 20015. Many of the injuries, the group said, were leg fractures.
The highest percentage of injuries happened to the youngest children, toddlers between 12 and 23 months, the study reported.
More than a third of the leg injuries occurred in the lower leg. And, the majority of the cases happened when a child's foot caught the edge or bottom of the slide, then twisted and bent backward while sitting on a parent's lap, the report said.
"Many parents and caregivers go down a slide with a young child on their lap without giving it a second thought," lead researcher Charles Jennissen, professor and pediatric emergency medicine staff physician at the University of Iowa Carver College of Medicine, said in a statement.
"And in most cases I have seen, the parents had no idea that doing so could possibly give their child such a significant injury. They often say they would never have done it had they known."
A child sliding down alone who might get his foot caught is less likely to be injured if he's not pushed by the forward momentum of a larger, heavier person sliding with him, the report said.
The researchers recommended adults and teenagers not ride down slides with children on their laps.
Clare told People that every playground should have that warning posted.
"I share this picture every year in hopes that the pain Meadow felt and the guilt that I still feel will save other babies and parents from the same," she told the magazine about her annual Facebook post.
"Don’t ever go down a slide with a baby on your lap. There is no SAFE way to go down a slide with your little."
Keetley wrote in her Channel Mum dispatch that she didn't even know her son's leg was broken until later at home when he climbed down from the sofa and fell to the floor because he couldn't bear his weight on his leg.
"Never did I think the doctors would come back to me and say his leg was broken.," she wrote.
“I wanted to share this video to raise awareness about how easily this accident can happen, and something so innocent can be so dangerous. You certainly don’t think or expect something like this could happen but it can so easily happen.”