A camera caught a smirking William attempting to contain his laughs.
After dazzling in a attractive cream V-neck dress by Alexander McQueen yesterday at Prince Louis' christening , topped off with a stunning Jane Taylor headband, the Duchess of Cambridge donned yet another lovely look by the British label.
The royal fab four made an appearance the 100th birthday celebrations of the Royal Air Force at Westminster Abbey on Tuesday.
The pair arrived at Dublin airport, with Harry wearing a green tie and former Hollywood actress Meghan in a dark green Givenchy dress that appeared to have been chosen with Ireland's national colour in mind.
Prince Harry and Meghan were among the 17 royals who flanked the Queen on the famous Buckingham Palace balcony, but the new Duke and Duchess of Sussex were in a prime spot right next to the queen with father-in-law Prince Charles and his wife Duchess Camilla of Cornwall on the other side.
Watched by the Queen and senior members of the royal family from the balcony of Buckingham Palace, the spectacle drew awe and applause from the crowd below.
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The couple met RAF servicemen and women, charity representatives involved in the RAF100 Appeal, RAF Veterans and Civil Service personnel involved in the Centenary celebrations.
Kate wore a pale mint Alexander McQueen design, while Meghan wore navy.
Harry and Meghan are in Ireland for two days and have an 11-strong entourage travelling with them.
They have a jam-packed itinerary in Dublin including meetings with the Taoiseach, Leo Varadkar, and Irish President, Michael D. Higgins, and visits to the likes of Croke Park.
We have to say, we think the new royal's minimalist look was strikingly similar to the famous black dress that Audrey Hepburn wore in her movie Sabrina.
The Queen watched Typhoons form the shape of the number 100 in the sky, before the Red Arrows, the RAF's aerobatic team, concluded the show, vapour-trailing the red, white and blue of the Union Jack flag.
DOMINIC LIPINSKI/PA WIRE/PA IMAGES The family at Prince Louis' christening on July 9.