Prince William has revealed the signature prank pulled frequently by his late grandfather which used to get him in "a lot of trouble" with the Queen.
In a preview for the BBC documentary Prince Philip: The Royal Family Remembers – which features interviews with more than a dozen members of the royal family – William explained that his grandfather's trick involved a tub of mustard and a lot of mess.
"He used to take the lid off and put it in your hands … and then he'd squish your hands together to fire the mustard on to the ceiling," he said.
"He used to get in a lot of trouble from my grandmother for covering most of the places we had lunch and things with mustard on the ceiling … He enjoyed those jokes, he enjoyed messing around the children and being a grandfather."
The mustard prank memory was also shared by William's cousin, Zara Tindall.
"I can't remember exactly what he says but he ends up slamming your hands together … It goes all over the ceiling," she said, while her brother Peter Phillips added: "I actually think the marks are still there."
Prince Philip died in April this year at the age of 99.
William also revealed that the Queen and Prince Philip had secretly "loved" awkward moments during their many public engagements.
"They've lived a life where everything has to go right the whole time and so when things go wrong, they both chuckle an awful lot," he said. "Everyone else gets mortally embarrassed. They love it."
In another preview for the documentary, Prince Charles declared that his late father "adored barbecuing" and was always the person in charge of the grill.
"He adored barbecuing and he turned that into an interesting art form," he told BBC One.
"And if I ever tried to do it he … I could never get the fire to light or something ghastly, so [he'd say]: 'Go away!'"
William also weighed in on his grandfather's cooking skills.
"Every barbecue that I've ever been on, the Duke of Edinburgh has been there cooking," William said.
"He's definitely a dab hand at the barbecue … I can safely say there's never been a case of food poisoning in the family that's attributed to the Duke of Edinburgh."