Sir Paul McCartney may be a world-famous rock superstar but to his grandchildren he's "Grandude".
The former Beatle has eight grandkids - six boys and two girls - aged from seven to 20.
He says they make music, watch football and dance together.
Now, their relationship has inspired his latest project - a children's book called Hey Grandude!
The title is of course a nod to the Beatles' Hey Jude. But the idea for the book came from the nickname given to McCartney by his grandchildren.
"One of my grandkids - who used to call me 'Grandad' - just happened one day to say 'Grandude' and it kind of stuck," McCartney explains. "So the other kids started calling me 'Grandude'."
McCartney liked it so much he started making up stories about a character called Grandude, who he pictured as a kind of retired hippie having adventures with his grandchildren.
"I imagined him as a sort of eccentric old guy.
"He's got a grey beard, a little bit of a ponytail, so he's a little bit groovy. He has a little hat and a bow tie.
"And he has this magic compass so when he rubs that you can go anywhere."
The book's fictional family take off on a whirlwind magical mystery tour. They ride horses with a cowboy in the desert, face an army of crabs on a tropical beach and dodge an avalanche while having a picnic up a mountain.
"Grandude isn't supposed to be me," McCartney is keen to clarify. "Because he's magic and I'm not."
Still, there are similarities. For one thing, they both entertain the grandkids by playing the guitar.
McCartney describes one night when the two youngest members of the clan were going to bed and he played them the classic Beatles ballad Blackbird.
"They were like, 'That's nice'. It's not like some famous guy doing Blackbird. It's just Grandad."
McCartney worked with the Canadian illustrator Kathryn Durst to create a picture book he hopes young and old will enjoy reading together.
He's also narrated an audio version of the book complete with an instrumental soundtrack.
He acknowledges there's more competition for children's attention these days with the popularity of video games and the internet. But he believes there's still a place for traditional storytelling - especially at bedtime.
"I like that - at the end of the day - sealing the day by reading to the kids," he says. "It's something I've always liked doing. And it's very satisfying when you see them nod off."
At 77, McCartney looks in great shape - full of energy and with a twinkle in his eye.
Dressed in a crisp white shirt and black jeans, he sits on a small sofa in his office with an acoustic guitar propped up beside him.
He's lived practically his entire adult life with a level of fame that's almost off the scale and yet somehow he still seems down-to-earth.
His room is packed with mementos from his long career. There's a Wurlitzer jukebox full of records he grew up listening to - Glenn Miller, Fats Domino, early Elvis.
Beatles box sets sit on the shelves. The statuette from the Wings Greatest album cover stands on the window sill. His desk is covered with family photos.
The singer has four grown-up children from his marriage to his first wife, Linda.
All have carved out successful careers. Heather is an artist; Mary's a photographer; Stella's a fashion designer; James is a musician. Four of the grandchildren are Mary's kids and four are Stella's.
He also has a 15-year-old daughter, Beatrice, from his second marriage to Heather Mills. McCartney says keeping up with the youngsters keeps him on his toes.
"They're football mad!" he exclaims. "And I kind of like football but not to the extent they do. They know every player. They can teach me everything about football."
When they're watching games together on TV, the veteran rocker admits his lack of sports knowledge can be exasperating.
"I'm like, 'Who's he?' The kids shake their heads and sigh, 'Grandad!'"
But they share his love of music and can all "hold a tune". Some sing in their school choir. One plays guitar in a band.
McCartney encourages them to make music for fun and insists it's "up to them" if they decide to do it professionally.
"Following me - for a McCartney kid - it's not that easy," he shrugs. "So I don't push them into it, or even encourage them to do it for a living.
"But just for pleasure I do encourage it. And we sit around and play together."
From an early age, the grandchildren were aware of McCartney's celebrity and used to seeing fans asking for autographs and selfies.
But McCartney says it's only when they see him in concert they realise the extent of his fame.
"I sometimes will say to the audience, 'My grandkids are here'. It's crazy for them. They've got this grandad up on stage rocking out."
After six decades in the spotlight McCartney shows no signs of slowing down.
He's still touring, performing three-hour sets for sell-out crowds in stadiums and arenas the world over.
He's also writing songs for his first stage musical - an adaptation of the classic Christmas film, It's A Wonderful Life.
As the interview ends, he picks up a harmonica and starts playing. Just for the fun of it.
Then he muses: "To me, I look at trees and I'm going, 'Do you realise they're pumping water at a phenomenal rate? And they're putting out oxygen!'
"Anyone else would go, 'Yeah okay, it's a tree Paul - get over it!' But I love the idea of seeing the wonder in the world because it is magical. And I think if you can see it then you have a better life."
Hey Grandude! by Sir Paul McCartney and Kathryn Durst is out now.