What is the secret behind Kate Middleton and Prince William‘s successful marriage? Well, one royal expert believes it is this unexpected side to their personalities that keeps the Prince and Princess of Wales’ spark alive — and it’s a vast contrast from their very proper public image.

William once described Kate’s sense of humor as ‘naughty’ and his own as ‘dirty,’ and claimed that it was one of the most important things that drew them together,” Christopher Andersen, author of The King: The Life of Charles III, told Fox News Digital. “They’ve certainly cleaned up their act for the children’s sake, but they can still make each other double over laughing.” That tidbit of palace insight gives fans a glimpse of what Kate and William are like behind closed doors — they might be conservative in private, but not necessarily in real life.