Margaret Mackie, 83, lives in a care home, and while she struggles to remember names, she stunned residents with a word-perfect rendition at the Christmas party.
A video of her performance with home worker Jamie Lee Morely, 31, went viral - so he decided to record the tune professionally and release it as a single.
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The song, which raises money for Alzheimer's Society and Dementia UK, is now sitting at number five in the Amazon best seller chart - above Lewis Capaldi and Dua Lipa.
In the iTunes Top 40 UK Pop Songs chart Margaret and Jamie are at number 27 - beating Ed Sheeran, Justin Bieber and Ariana Grande.
Margaret, who used to work in PR, is a mom of two.
She has no grandchildren.
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Her daughter Mairi Hunter, 55, said:
"We put mum in another care home before the one she is in now and she went downhill really fast. But since we moved her she has just bounced back. We have got our mum back with Jamie and the singing."
"She has always loved to sing and as she can't read her books or the paper anymore, the singing has just taken over. Seeing her singing with Jamie was incredible, they get on so well and sing around the home a lot, but to actually see them together was lovely. She thinks so much of him."
Of the song's success, Margaret remarked:
"It is just amazing. I feel so happy about everything. It's great we can raise all this money for charity."
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Proud Jamie, a singer, added:
"It was the first time that she'd been in the studio, but was like she had done it a hundred times before. She's got such an amazing voice. The fact that she can do that while living with dementia is mind blowing. It's just crazy."
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He remarked:
"It's like the music is the therapy for her. You just see the change of the look in her face when she starts singing or she hears music that she likes. You can just see it in her."
"Music is my passion. It can change your mood instantly and with her it's exactly the same. She comes alive when she sings and she looks so happy, it is beautiful to see. It is the power of music and proves how amazing it can be for people."
Margaret moved into Northcare Suites Edinburgh care home just weeks after it opened last September, and Jamie said she is always singing and dancing.
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Jamie, who is also a part-time singer, was asked to arrange the Christmas show for the residents and asked Margaret to join him for a duet.
He said:
"You never know how they are going to react. She is living with dementia. All I knew was that she loves music and we always sing 'My Way' together and I just thought it would be a great moment."
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He added:
"My granddad passed away - he had Alzheimers and that was his favorite song so it had a connection to all of us. I just wanted to enjoy the moment with her and get her up there, and she sang every word absolutely amazingly."
"The whole audience erupted. The whole place was in tears. Everyone just burst into tears - she stole the show. The response was unreal. The whole place was crying, we got a standing ovation, everyone was clapping. Everyone was blown away."
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It was filmed by her daughter, Mairi, who put it on social media, and within 24 hours it had gone viral.
They headed into the studio in December and Jamie said it took just 45 minutes and two takes to record.
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Jamie gushed:
"For someone with dementia, it fascinates me how amazing music makes her feel. She becomes a different person. It's just incredible. She has definitely got the music in her blood."
"It's so refreshing to see someone living with dementia acting the way she does. She is an inspiration to so many people - especially me! I just love her."
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You can listen to the version they recorded in the studio below: