Chelsea Dunn was one of the bright lights in her family -- athletic, opinionated and talented. The 13-year-old was the classic all-American kid growing up just outside Boise, Idaho. A twin to her brother, Hunter, and a big sister to 5-year-old Balee, Chelsea was already mapping out her future. Her parents, Joe and Tammy Dunn, were proud of their vibrant young daughter, who had been accepted to a new charter school earlier this year, and dreamed of becoming an artist.
On the night of April 14 the family's ideal life took a tragic turn. Chelsea headed off to bed while her family was watching television and her mom gave her a kiss goodnight.
When Chelsea didn't come out of her room for breakfast the next morning, her father asked Hunter to get her out of bed.
Hunter discovered a ghastly scene in his twin sister's room. Chelsea was hanging from her closet door with a belt around her neck. She appeared to be dead.
Hunter, stunned, told his father to look in the closet. "I heard Joe screaming. I went to the entryway to the door, and I looked over and saw her. And I turned away really fast and ran to the phone and dialed 911," Tammy told "20/20's" Deborah Roberts.
Police and paramedics rushed to the house only to discover that Chelsea had died the night before, minutes after saying her last goodnight.
The devastated family was haunted by questions. How could their sweet, lively girl take her own life and why? As the family searched Chelsea's room for clues they made a startling discovery.
They found a note Chelsea had written to a friend. She wrote, "I love doing that pass out thing. You wake up and you forget what happened. It comes back though you're all tingly."
"I said that's it. This is exactly what we're looking for. This explains everything," Laura Cooper, a close family friend told "20/20."
Joe and Tammy were shocked by the note. "My husband had gone in to talk to her brother and he had confirmed that she had confided in him that she was playing the game with friends in the PE locker room," Tammy said.
But what was this mysterious "pass-out" game? Tammy had never heard of it, but Joe had. "I saw kids do that when I was in the sixth grade at the elementary school I went to. I didn't have the slightest idea that it would still be going on today," he said.
Apparently, it's a popular game with kids around the world and it goes by a lot of names. "They can be known as space monkey, space cowboy, knockout, gasp, rising sun. In Ireland, it's known as the American dream game," said Dr. Thomas Andrew, an expert on the dangers of this high-risk game.
Andrew says children play it by squeezing a friend's chest or neck to cut off the flow of oxygen. "While the brain is deprived of oxygen, you'll get this sensation of light-headedness. Perhaps numbness and tingling. And if all goes as planned, the pressure's then released. Blood goes torrenting up those carotid arteries and it goes into the brain, and you have this big rush," he said.
That rush is what children seem to crave. Some want it so much they're now often playing the game alone, using shoelaces, ropes, dog leashes, bed sheets and belts. Andrew says the result can be fatal -- and easily confused with suicide.