We finally know the reason the Royal family are laughing in Prince Charles' birthday photo.

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If you want a concrete example of just how much the British Royals have lightened up over the past few decades, take a look at their official portraits. Once staid and stiff, the Windsors are now doing their best to project a more, erm, relatable public image.

The family photo taken to celebrate Prince Charles’ 70th birthday is a prime example.

1. Princes don’t even bother doing up top buttons anymore. Or wearing ties. (They’re cool now, see?)
2. Spouses can have actual physical contact.
3. The children don’t look like ghosts.
4. They’re all laughing.

Not just a polite chuckle, but – in Meghan, Duchess of Sussex’s case at least – a proper bent-over, out-loud laugh.

The question on everybody’s minds after the images were released on November 14, was what gave the Royals the giggles? Did the Queen wander past in her trakkies? Did little Prince Louis let one rip? Did they suddenly notice the whole awkwardly colour-coordinated outfits thing?

Well, it turns out Maria Borrallo, the nanny to the young royals, was responsible for the hilarity.

An unnamed person, who was reportedly present at the September shoot, told Us Weekly that the Royal's answer to Mary Poppins was standing beside photographer Christopher Jackson, "pulling funny faces" in an effort to get a smile out of Princes George and Louis and Princess Charlotte.

Whatever she did, it seemed to appeal to the grown-up's sense of humour, too.

According to the publication, the shoot only took 15 minutes; because of the children, “everyone had to work fast.” Though Prince Louis, especially, was apparently very well-behaved: "He is the most adorable, placid little boy you will ever meet!” the source said.

Borrallo, a native of Spain, has been with the Duke and Duchess of Cambridge since 2014, when she swooped in to assist with childcare for a then-eight-month-old Prince George.

In the years since, she's accompanied the children to several high-profile engagements including Princess Charlotte's christening, Prince Harry and Meghan Markle's wedding, and Princess Eugenie's wedding to Jack Brooksbank.

According to a recent article by People, she's “firm and strict but she is very loving and soft with [the children] too.”

(And clearly also hilarious.)

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