Back in November 2018, a documentary about Prince Charles aired on the BBC in celebration of his 70th birthday.
Called Prince, Son and Heir: Charles at 70, the programme featured a number of different members of Britain's most famous family, including Prince Harry and Prince Wiliam.
The brothers shared a number of behind-the-scenes details about their lives, with William revealing that he'd like his father to spend more time with George, Charlotte and Louis and Harry admitting that he still refers to the l ate Princess Diana as "Mummy".
The Duke of Sussex, 36, also opened up about asking his father to walk Meghan Markle down the aisle at their wedding and a bizarre habit that he shares with both his dad and older brother.
According to the man himself, they are all obsessed with turning lights off - and the habit stems from the Prince of Wales.
Speaking in the documentary, Harry said: "He's a stickler for turning lights off.
"And that's now something that I'm obsessed with as well, which is insane because actually, my wife goes 'Well, why turn the lights off? You know it's dark'.
"I go 'we only need one light, we don't need like six', and all of a sudden, it becomes a habit and those small habit changes he's making, every single person can do. And I think that's one of the key lessons certainly that I felt that he taught us."
William completely agreed with his brother, adding: "I know I've got serious OCD on light switches now which is terrible."
And turning the lights out isn't the only thing Charles has taught his sons to do to help the environment.
The pair also revealed that he has always encouraged them to litter pick when they were out and about.
"I used to get taken the mickey out of at school for just picking up rubbish," Harry admitted.
"I don't go consciously looking for it. When you go for walks anywhere, if you see something, it stands out, you pick it up.
"It's like I've literally done this because I'm programmed to do it, because my father did it. Actually, we should all be doing it."
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