Princess Charlotte's favorite joke has gone viral on TikTok after a fan shared a clip of Prince William telling it on a royal visit.

The 9-year-old royal and her brothers, Prince George and Prince Louis, are among thousands of U.K. children heading back to school this week after a long summer break.

And a fan of the princess shared what she described as the "cutest joke," told by William during a visit to St Michael's Church of England High School, in Sandwell, England, on April 25.

From left: Princess Charlotte is seen at Centre Court during the men's final of the Wimbledon tennis championships in London, England, on July 14, 2024; and Prince William laughs during a friend's wedding at Chester... From left: Princess Charlotte is seen at Centre Court during the men's final of the Wimbledon tennis championships in London, England, on July 14, 2024; and Prince William laughs during a friend's wedding at Chester Cathedral in England on June 7, 2024. The father told his daughter Charlotte's favorite joke, and the clip has gone viral. Karwai Tang/WireImage and Max Mumby/Indigo/Getty Images

Pupils asked the Prince of Wales to share a "dad joke," and he replied: "I think what I'll do, is I'll say a joke which is Charlotte's favorite at the moment."

William then told a "knock knock" joke, answering "interrupting cow" when asked "who's there?" As the pupils tried to reply "interrupting cow who?" the royal cut them off by saying "moooo," causing a ripple of laughter.

One of the children, 12-year-old Freddie, later gave an interview to BBC Radio: "I did laugh. It was a little bit funny. We did laugh with him."

During the visit, William also poked fun at Jack Whitehall—a famous U.K. comedian who starred in Jungle Cruise and Clifford the Big Red Dog. "I've been asked to produce a dad joke, so I'm kind of trying to channel Jack Whitehall; most of his jokes are pretty dad-like," the prince said.

On X, formerly Twitter, the comedian later commented, "there goes my knighthood," before adding: "An outrageous shading from the future King."

William spent the summer mostly out of the limelight with Princess Kate, Charlotte, George and Louis.

It will have been a welcome break for the family after a difficult 2024 in which Kate was diagnosed with cancer and began chemotherapy.

William and Kate did step briefly back to work to record a good-luck video for Britain's Olympics team, in which he sported a beard, but otherwise the couple have had weeks to relax and unwind.

The prince is now preparing to go back to work, though it remains unclear what the rest of Kate's year will look like.

She has now passed six months of chemotherapy, which she began in late-February, but has not formally returned to the office.

And there are some questions that remain outstanding, including whether Kate will be able to host her annual carol service, Together at Christmas, at Westminster Abbey in London, in December.

Jack Royston is Newsweek's chief royal correspondent based in London. You can find him on X, formerly Twitter, at @jack_royston and read his stories on Newsweek's The Royals Facebook page.

Do you have a question about King Charles III, William and Kate, Meghan and Harry, or their family that you would like our experienced royal correspondents to answer? Email royals@newsweek.com. We'd love to hear from you.

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