When visiting two future kings at home, it may be difficult to come up with a suitable gift to mark the occasion.
Fortunately, Sir David Attenborough had just the thing in his pocket: the tooth of a 23-million-year-old giant shark he found on holiday.
Sir David personally presented Prince George with the treasure, which he found in the 1960s during a family trip to Malta.
The young Prince, who like many small boys has a fascination with fossils, appeared thrilled with the gift, and is shown beaming in a photograph and studying it proudly.
Attenborough, 94, visited Kensington Palace this week for an outdoor screening of his upcoming film with the Duke of Cambridge.
But the opportunity to see their broadcasting hero proved too much of a lure for the whole family, and the Duchess and three Cambridge children joined the party in the garden after school.
A photograph capturing the moment they met, while adhering to social distancing, shows Princess Charlotte, 5, with her hands clasped to her face in apparent delight, as she stares at Attenborough's familiar face.
The Duke, holding a copy of the naturalist's latest book, greets him like an old friend, while the Duchess laughs at her children's reactions.
Prince Louis is dressed smartly for the occasion, and was soon absorbed in a show-and-tell of the ancient shark's tooth.
Prince George, seven, is known to have a fascination with dinosaurs, with his mother once telling a group of children during a Natural History Museum that he "loves the T Rex because it's the noisiest and the scariest."
The visit was a long time coming for the three children, who have all watched his television programmes.
On a previous encounter with Sir David, the Duchess told him: "The children were very upset that we were coming to see you and they weren't coming, they're massive fans of yours."
The Duke and Sir David have been working together for some time on their shared passion for the environment, appearing on stage together at Davos, for a joint unveiling of the ship once named "Boaty McBoatface", and at the premiere of his documentary Our Planet.
Their partnership has now extended to the Duke's Earthshot Prize, due to be formally launched soon, with Sir David providing the voiceover for its introductory video.
On Thursday, he paid a visit to the Cambridges at their London home, for a special screening of his new film, David Attenborough: A Life On Our Planet, said to be his "most personal project to date, in which he presents some of the most defining moments of his career as a naturalist and the devastating changes he has seen".
The Duke and Sir David were photographed watching the film on a cinema screen beneath traditional red velvet curtains, sitting in directors' chairs on which they swapped the names on the back for their amusement.
A source said the Duke had thoroughly enjoyed the film, thinking it was "one of Sir David's best yet" and noting that its optimistic message is "very much in line" with his own views.
"It says 'we've got a chance to fix this'," said the source, of the films' approach to saving the planet. "That very much chimes with the Duke's view that we've got an opportunity to get this right."
Meeting the children, Sir David brought along the tooth of a giant shark carcharocles megalodon - or "big tooth" - which he found on a family holiday to Malta in the late 1960s embedded in soft yellow limestone from the Miocene period 23 million years ago.
Carcharocles is believed to have grown to 15 metres in length, which is about twice the length of the Great White, the largest shark alive today.
The veteran broadcaster has a long and warm relationship with the Royal Family, which has included producing the Queen's Christmas speeches for the BBC in the late 1980s and starring alongside her in a documentary about the Buckingham Palace trees in 2017.
In 1958, he gave the young Prince Charles and Princess Anne a tour of his television studio with the help of his pet cockatoo.
Sharing details of the film, a spokesman for the WWF said of the Duke and Dir David's relationship: "With a shared passion for protecting the natural world, they continue to support one another in their missions to tackle some of the biggest environmental challenges our planet faces."
David Attenborough: A Life On Our Planet is described as serving as Sir David's "witness statement for the natural world".
It will premiere in cinemas on Monday, 28 September before launching on Netflix a week later.