It was the most fleeting of moments, but it said everything about the Duke and Duchess of Cambridge's relationship and the reason their tour Downunder has been such a success.
Arriving at Sydney Opera House on Wednesday for their official welcome to Australia, the couple walked up the steps to the building, admiring the view of the Harbour Bridge bathed in autumn sun.
Wrapped up in the moment, Kate failed to register the crowds behind screaming for her to turn so they could get a picture. William, ever the professional, touched her hand as a sign to turn around, prompting wild cheering from the throng as she flashed a perfect smile.
What a difference from the corresponding tour by the Prince and Princess of Wales in 1983, when Charles could not understand Diana's rock-star popularity and quickly came to resent it.
William not only understands his wife's popularity, but actively promotes it. He knows that he comes a distant second to the Duchess (and both are outshone by Prince George), but he is clearly comfortable with that, partly because of his obvious love for her.
When she spoke at a children's hospice in Manly William, playing cheerleader, even let out a "Woo" as she was introduced by its chief executive.
Having followed the couple on all three of their foreign tours, I have been struck on a daily basis by just how unstuffy they have been here.
They have posed for endless pictures, competed at cricket and publicly poked fun at each other throughout their time in New Zealand and Australia.
Kiwis and Aussies are never ones to stand on ceremony, and the Duke and Duchess have fitted right in. The crowds that have gathered to see them have also been delighted at their willingness to pose for pictures with the public - once considered an unseemly breach of royal protocol - leading to a proliferation of "selfies", many of which have turned up on Twitter and Facebook.
Every royal marriage is a combination of the personal and the professional. Not only do a royal couple have to love each other, but they have to be a successful public double act, and if there are any cracks in the personal side, it will soon show in public. The Queen and the Duke of Edinburgh, for instance, clearly have huge affection for each other, though they show it in subtle glances and wry remarks rather than physical contact and body language.
Even the most battle-weary royal hacks and photographers have been touched by seeing William and Kate at play, such as their impromptu cricket match in Christchurch where the Duke bowled a beamer at his wife's head (with a light rubber ball) and roared with laughter as she wagged a finger at him to remonstrate.
After he had poked fun at her dress sense, saying her yellow dress made her look like a banana, she got him back in spades in Sydney by suggesting he should wear a toupee.