Do you want to know his secret?
Paul McCartney whistles as he walks into the room where our interview is taking place. It's not a recognisable tune, but it's pleasant nonetheless -- and you just know there's a good chance he's making it up as he goes along.
This is a man who can literally create works of genius in his sleep -- 'Yesterday' came to him in a dream -- and that makes it all the more surprising that, for his latest project, he's opted to record what's predominantly a covers album.
Kisses on the Bottom -- which takes its peculiar title from a line in the opening track, Fats Waller's 'I'm Gonna Sit Right Down And Write Myself A Letter' -- is a loungey collection of the songs of McCartney's youth and, he explains, the songs that laid the foundations for his later work.
"I grew up with this style of song. And I like that style very much, because it's well made, it's like a fine piece of jewellery -- all those songs from Gershwin or Cole Porter. If you're a songwriter, they're very well crafted. And I grew up with that sense of craft," he says.
"Then rock'n'roll happened, I met John (Lennon), and we started writing songs. So we went off into the rock'n'roll direction, but this was always in the background. One of the things about Beatles songs, if you analyse them, is that they're a little bit different from the normal rock'n'roll songs.
"They've got this . . . this background. So, if you think about some of the songs like 'Here, There and Everywhere', it's got a little introduction that rock'n'roll songs never had. But we said 'Oh, it'd be nice to write a little introduction, a little set-up, or a middle where it goes to different chords'."
There are few people better placed to talk about what makes perfect pop -- and it's a subject in which McCartney revels.
"I think a good melody, a memorable melody, really helps. People find themselves whistling it, or singing it in the shower, and they think: 'What's that?' And that's what's important, really, in a good pop song," he says, before comparing his craft to yet another physical trade.
"Technically, it's got to have structure; it's got to be like a well-made piece of furniture. It's got to hang together, to be well made. Then, I think, it's got to mean something -- and with the good songs, you can't help it. They just mean something."
While this affinity for all things old-timey has always been obvious, with Beatles tracks such as 'Honey Pie' and 'Your Mother Should Know' featuring obvious tips of the hat, this is McCartney's first time making a conscious effort to replicate the style of the era. And with the album's standout song, he has triumphed.
'My Valentine' is the type of song that die-hard Lennonists love to scoff at; a honey-sweet affair, painstakingly orchestrated and delivered with McCartney's baby-soft vocals -- it's his finest composition in some years, and fits in seamlessly with the 'authentic' oldies on the album.
"That song was actually really easy to write," he says. "We (Paul and new wife Nancy Shevell) were on holiday in Morocco, and it was raining. I said: 'It's raining -- it's going to spoil it.' But she said: 'It doesn't matter. It's great! We're having a good time.'
"So there was a piano in the hotel that the guy in the evening would play all the old songs on. So I was in that mood, and I just went there in the afternoon while the waiters were clearing up, and I played. And this song came out," he says, before singing the song's first two lines: "What if it rains, she didn't care."
"It sort of wrote itself, which all the best songs do. You can't believe it's coming out so easily, you know?"
Easy for you to say, Paul. But does it come that easily for today's crop of talent -- and are there any signs of a 21st-Century successor to The Beatles?
'Now it's difficult for bands, because so much has been done. It's very difficult to do what The Beatles did, you know, no matter how good they are. I think there's a lot of good bands, though -- Coldplay are a good, modern band. But I don't think they're as good as the Beatles! Foo Fighters are great . . . but they're not as good as The Beatles!
"I think it really would be difficult to do what The Beatles did, it's very difficult to get four people so in tune. When you think about it, any one of The Beatles could have had his own band. But what happened was you had the four of us in the one group -- and only four -- and I think that was a bit magical.
"It would be difficult to recreate that. I wish everyone well, and I hope someone can do it . . . but they won't be as good as The Beatles!"
It's a refreshingly optimistic attitude from someone who turns 70 in June -- and it's somewhat difficult to imagine what keeps him ticking.
"Sex and drugs. No, wait a minute, I don't mean that! No, you've got to love what you do. And if you're lucky enough to love what you do like I am -- and you are -- then that's the secret."
Kisses on the Bottom by Paul McCartney is out now