She jumped on her father's forklift truck, only for the beamer to come down on her back. She jumped off, ran a few steps, then slipped in compacted snow.
"I felt all my toes curl, and then it went all fuzzy, like all the way up my legs," says Sammi. "It felt like something really warm was holding me tight, and then my legs kind of released. I knew that was the last time I was going to feel my legs.
"I knew right away I had broken my back."
The 18-year-old from Gordon has crafted glorious triumph from that catastrophe less than four years ago, winning three gold medals this week at the IPC European Championships in Swansea - three of seven won by four Scots, helping Britain to second on the medal table.
Reid, who has won the 100, 400, and 800 metres this week, is in the same T53 category as Britain's iconic Paralympian, Baroness Tanni Grey-Thompson. "She holds all the British records in my class," says Kinghorn, "and is my role model - the person I want to be. And it was her husband, Ian Thompson, who opened my eyes to sport and brought me into wheelchair racing. That was at the Spinal Unit Games. He told me I could be really good, so I gave it a shot. I've spoken to Tanni just a little. She works very hard and isn't around that often, but knows I want to steal her records. It's all because I want to be just like her. I think she is pretty awesome."
Kinghorn is less than a second outside these UK records, but may need to beat them to fulfil dreams of world gold next year in Doha, and at the 2016 Paralympics in Rio de Janeiro. "I hope to do the 100, 400 and 800," she says.
Four summers ago, aged 14, it was so different. "I'd not long started high school. Sleepovers were a big thing. I'd hang around with my friends and worked on the farm, outside a lot. I still do everything most normal 18-year-olds do, but I can't go out every weekend, party, and drink a lot. My friends are very understanding. I'm a full-time athlete, so instead they come over and we'll have a movie night. They were all here in Swansea."
She was considering a career in zoology: "working with animals. I knew I wasn't smart enough to be a vet. I planned to travel and then try zoology, but I've changed my mind. I'm considering child psychology, but I also like the media. The sporting world might open doors."
She insists, however, that life has changed little. "I do more than before, to be honest. Party in the middle of a grass field? 'Right, we'll carry you'. Top-floor flats? No lift? 'We'll carry you'.
"There are no limits. It's the attitude of the person in the wheelchair, not the wheelchair itself. Life is too short to moan and not do things. Anything my friends are doing, I want to do too.
"I believe everything is possible. I don't think there is anything I can't achieve. On the farm I help with lambing, and move sheep around. I have my own quad.
"Obviously, a big aim is gold on the world stage and a world record. I know what I want, but you don't know who is going to be out there."
Hockey and gymnastics were formerly her main sports. "I wasn't fantastic. It was a social thing. I was about to take up coaching gymnastics. I did ballet and loved it, but I wouldn't have said I was a very talented sporting child."
She spent six months in Glasgow, in rehabilitation. This was extended, learning to use calipers to walk up the aisle at her aunt's wedding. Glasgow's Red Star Athletics club, where she met its co-founder Ian Mirfin, was a revelation. He is scottishathletics' event lead on disability and is now her coach.
She "hasn't a clue" whether World Class Performance funding might improve after this week: "I was not expecting this."
The Lottery "and nice people in my community, and a company called Mattioli Woods" have helped her. Her latest wheelchair - called Poppy - cost £6000. "My shape, everything, has changed, and my legs were getting smaller."
She trains up to five hours a day: road in the morning, track or rollers in the evening. "If it's chucking it down, I go on rollers in the garage. It's weird cranking up the miles while looking at myself in the mirror. I normally have the music up pretty high. I listen to everything - dance, Tina Turner - anything."
Tomorrow she contests the 1500m at the Birmingham Games. The field includes the world No.2. Youngest of Scotland's Commonwealth team, she was fifth in this event at Hampden. "Birmingham is not a fast track," she says, "so it will be about the experience of racing in the pack."
Then it's off to Lanzarote for 10 days with her boyfrend. "A couple of years ago I never thought any of this would be possible. It's pretty awesome. I thought I'd be stuck in a bed for life.
"I'd probably settled for something worse when I got my chair. That was absolutely amazing - 10 times better than being stuck in bed."