Madison Lambe, 12, asked the Duke of Cambridge, 31, for a photo when he was walking from church on Christmas Day with wife Kate in Sandringham.
And what was William's surprising response? "You can't beat a good selfie on Christmas Day."
Young Madison told the BBC she had brought some flowers along with her, and when he passed she made her approach.
"Then he said, 'Right, you can't beat a good selfie on Christmas Day,' " she reports. "I didn't really expect it. It was a really good experience."
Back in the summer, even William's 87-year-old granny, Queen Elizabeth, got in on the act of surprising a royal fan by posing with her at an event on the grounds of Buckingham Palace.
After spending the New Year with his Middleton in-laws in Bucklebury, William is preparing for his return to college next week, when he will begin a 10-week agricultural course at Cambridge.