Britain's Kate Duchess of Cambridge with Prince Harry visit Bacon's college in London. Photo / AP
Whispering conspiratorially in her ear on stage at the Queen's 90th birthday street party. A good-natured tease about her skyscraper heels that might have got stuck in a grille as they walked up the aisle in St Paul's for a thanksgiving service for the Monarch.
And not forgetting the chat that reduced her to teeth-flashing, eye-crinkling giggles as they sat together during the same service, while Prince William stared blankly in the other direction.
It's becoming rather hard to ignore how well Prince Harry gets on with the Duchess of Cambridge. Whether it's at the London Olympics, weddings, Trooping the Colour or even sombre events like the World War I centenary commemorations in Belgium in 2014, Harry and Kate can often be seen side-by-side.
Harry is, of course, a joker, and likes to give humorous asides during some public events, like his grandfather. And Kate is a polite, responsive soul, not one to ignore others' attempts to make her laugh. Regardless, the closeness of their relationship is all too apparent.
Harry has plenty of female friends - including cousins Zara, Beatrice and Eugenie - but he is probably closer to Kate than any other woman on the planet.
While cheeky online commentators have suggested there's a flirtation going on, one source insists: 'That's not it, although he does have a knack of making women, including Kate, feel good about themselves, commenting on what they are wearing, for example.
'But as Harry once said, Kate is like the big sister he never had.' Even, occasionally, a mother figure.
'While William and Harry get on with Camilla and are delighted to see their father happy, they don't really confide in her,' said one source. 'She can't replace their mother, and wouldn't expect to.'
And so, into the emotional vacuum Diana left behind for both her sons, stepped Kate. She has been both a trusted counsel and something of a partner in crime for the persistently single Harry.
Theirs is a genuine friendship, founded on nights spent in front of Game Of Thrones boxsets and companionable suppers. From giving him cookery lessons to offering sensible romantic advice, Kate has become Harry's lynchpin.
So what does Kate find so appealing about Harry? A willingness to lark around, for one thing - something the usually serious William isn't often seen doing.
'Kate has quite a different relationship with Harry than she has with William,' says the royal insider. 'Harry appeals to her sillier side because he is carefree, footloose and loves banter. He finds things funny about people and says so.
'William, partly because of his position, partly because of his character, plays it safe like his father, whereas Harry is free and easy, with a "let's have the whole bottle" sort of attitude. Kate also adores the way Harry plays with her children - he is very good with them, a charming big kid and silly uncle.'
Unlike most vigorous, fun-loving young people, a single Prince - and, indeed, a future Queen - have to be careful whom they trust. And, alongside William, Kate is as safe as it can be for Harry, and vice versa. Their friendship has a history that extends through Harry's adult life.
Kate started dating William in 2003, after meeting him at the University of St Andrews. Soon after, she met Harry.
He was still a teenager, about to leave Eton, and was deciding what to do with his life. After an extended gap year, when he travelled to Australia and Lesotho, he finally went to Sandhurst in 2005.
According to a source at St Andrews: 'From the earliest days, Kate always maintained an open-door policy to Prince Harry.
'While they were students she always made him feel welcome at the four-bedroom cottage they rented on distant royal cousin Henry Cheape's estate, Strathtyrum, just outside St Andrews. The three also spent weekends together at the cottage, Tam-na-Ghar, on the Balmoral estate.'
Another insider said: 'Cynics - the sort that said she deliberately swapped Edinburgh University for St Andrews when she found out William was going there - might say she knew that getting on with Harry was key to her relationship with William.
'Harry, although he can be moody, over-sensitive, and unreliable to his girlfriends, is otherwise enormously good fun. Everybody likes him. Why shouldn't Kate, too?'
When Harry was in the Army, he visited William and Kate in Anglesey, in their five-bedroom rented farmhouse on Sir George and Lady Meyrick's estate.
Here, I'm told, the trio enjoyed local pubs, high teas and hearty suppers after long walks on Anglesey beaches away from the public eye. A 'strong, sisterly bond was cemented with the at-times lonely Harry', says this source.
Years before William proposed, Kate was already exerting her benign, civilising influence on both brothers. In a gentle, nurturing way, and closeted in the privacy of Anglesey, a source tells me Kate gave Harry 'cooking tips, so he would eat more than the Etonian-style fry-ups of which he was so fond'.
In London, while William was learning how to fly, he and Kate would retreat to the flat in Clarence House he shared with Harry. Kate organised the cooking - simple, nursery food such as sausages or roast chicken - and Harry was generous with the drinks.
'From day one Kate made a massive effort with Harry. But it was a natural thing for her because she grew up in such an inclusive family, the "en masse Middletons" and all that,' says another source, referring to the nickname the Middletons were given because the whole family moved together in a tight pack.
If Harry and Kate were confidants before William married her, they are even more so now. When the trio's living arrangements changed, they became neighbours on the Kensington Palace compound, with Harry's move to Nottingham Cottage and William and Kate's to Apartment 1a, mere yards away.
'Harry is totally at ease popping over to the Cambridges' gym to work out next to Wills, and he's used to seeing Kate in her cashmere dressing gown reading to Charlotte, or joining in cosy family suppers, eaten from their laps,' says one royal insider.
'Harry has been encouraged by both William and Kate to see as much of George and Charlotte as he likes. He is a great supporter of George to be as adventurous as possible, so football in the KP garden is a must, along with walking Lupo the dog.
'There are constant japes with the children. If he can return them muddy, Harry sees it as a job done!' When it comes to Harry's girlfriends, Kate is subtle, while simultaneously holding strong opinions.
'Like the Queen, she wouldn't state her point of view, more ask questions,' says one acquaintance. 'If she didn't think his former girlfriends such as party-loving Chelsy Davy or actress Cressida Bonas were real runners, it would be because she knows more than anyone what marriage into "The Firm" [as Prince Philip describes the Royal Family] actually entails.'
Friends believe Cressida and Harry came within a 'whisker' of getting engaged, but she wasn't willing to give up her theatrical career.
'Both Kate and Camilla have been working through their contacts books to look at "prospectives" for Harry,' says another royal source.
'Discretion is key. Candidates must also be athletic, love animals, retain high levels of stamina, be reasonably well educated if not fiercely academic, have no unguarded "mouthy" moments, be fun-loving, able to travel effortlessly and have no medical problems. A tall order in one girl!'
While Kate would like Harry to find a bride, the potential addition of another person into their tight unit poses its own problems.
For the moment, Harry being single 'works just fine' says the royal source. 'It's not about competition, but turning their threesome into a foursome will be a daunting prospect, whoever it is. The status quo does rather suit Kate because she can be leader of the pack - and doesn't have to accommodate a stranger. They're used to their cosy trio. It's hard to imagine anyone else joining them.'
While he lacks a girlfriend, and all too aware that Harry has lost the 'strong protective arm of the Army and close camaraderie of his colleagues there', says one friend, William and Kate have been at pains to make Harry feel he has 'a pivotal role' in their extended Royal Family life.
Just look at the ongoing success of his Invictus Games, the charities he assists - including Help For Heroes, WellChild and, of course, Sentebale, the one particularly close to his heart as it was set it up in his mother's memory, to help children in poverty and with HIV.'
Indeed, the young royals became a trio at work, too, when it was decided that William and Harry's office would also serve Kate, rather than she and William splitting off and leaving Harry to run his own royal operation.
On a more personal level, this helps Kate deal with 'The Firm'. When it comes to Palace politics, Kate and Harry are allies because 'she is a fan of his and he can do no wrong in her eyes, though there are times she thinks he's been led astray [for example, when he was pictured naked in Las Vegas in 2012]. He won't hear a word against her either,' says one acquaintance. Indeed, Kate tries to keep a protective eye out for the trusting Harry. Whereas Charles and William can be angry with Harry when he makes public misdemeanours, Kate is more likely to blame the crowd who surround him.
For example, she has long been wary of royal friend Guy Pelly, whom she saw as a wild card in the Prince's pack. Cressida Bonas agreed, splitting up with Harry when he went to Pelly's wedding without her. Pelly, now though, is apparently back in the royal circle as his American heiress wife Lizzy Wilson has been warm and diplomatic to William and Kate.
Equally, Harry helps Kate because 'with one off-the-cuff remark he can defuse any row. And he hasn't got the attention span for Palace and family politics although he shares their dread of the Press. But he is no grudge bearer - quick to anger, quick to defuse - where William can be obstinate to the point of lunacy.'
However close William, Kate and Harry are, they cannot agree on everything. When it comes to the Yorks - namely Harry's friendship with Beatrice, Eugenie and their parents, and his regular visits to Chalet Helora, Prince Andrew's £8 million skiing retreat in Verbier - William and Kate will always side with Charles and, like him, would prefer not to have the Yorks too close.
In 2011, after William proposed, with some prescience, Harry said he would take Kate under his wing, 'or she'll be taking me under her wing probably'.
With his peace-making instincts and humour, and her sisterly wisdom, it seems to be a case of both.
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