Prince William and Middleton
Kate Middleton and Prince William shows rare PDA during interview for "Blue Peter." Pictured: Prince William and Middleton pose for the official tour portrait for their trip to Canada and California in the Garden's of Clarence House on June 3, 2011 in London. Getty Images/Chris Jackson

Prince William and Kate Middleton rarely hold hands or show PDA in the public, but sometimes they do.

Last week, the Duke and Duchess of Cambridge were interviewed for BBC's "Blue Peter." During the interview, Middleton touched Prince William's knees as they playfully joked around. Per Daily Mail, the pregnant royal mom's gesture reminds them of Prince Harry and Meghan Markle's "loved-up engagement."

"That's very touching. Thank you very, very much," Prince William said after receiving his badge from Radzi Chiny and Lindsey Russell. "We've still got a long way to go yet but we are getting there. A shift is happening and we will hopefully crack a problem that should have been cracked a long time ago."

Prince William also joked about his grandmother having a badge. "'We'll compare!" the Duke of Cambridge said.

Prince William and Middleton usually keep their hands off from each other in public and on royal engagements to appear professional. But prior to their PDA last week, there were a few times when the couple broke the protocol.

In 2012 Olympics, the Duke and Duchess of Cambridge shared a hug when the British team won the gold medal over France in track cycling. Also, in 2016, the couple joined Team Heads Together at a London Marathon Training Day in a relay race against Prince Harry. Middleton came last and Prince William reassured her with a consolatory arm around her shoulder.

Unlike the Duke and Duchess of Cambridge, Prince Harry and the "Suits" actress have been open about showing their affection towards each other in public. In fact, the newly engaged couple was seen holding hands on several occasions. When they made their first public appearance in Toronto during the 2017 Invictus Games, the lovers were photographed hand-in-hand.

According to body language expert Robin Kermode, hand-holding is a deliberate act to show that one's relationship is solid. Prince William and Middleton "do not feel the need to prove their love" to the public.

Kermode also stressed that Prince William and Middleton may have taken this after the monarch who also rarely holds hands with Prince Philip. "The Queen rarely holds hands with her husband in public and this seems to have set an unwritten precedent for the other royals," Kermode explained.

"It is very clear that William and Kate are emotionally close, however, they do not seem to feel the need to prove their love – particularly when on official state business. They come across as a strong, independent couple and while they are always on show, they do not feel the need to show their love for each other to the world," he added.