Prince William cracked a ‘dad joke’ during his surprise visit to a school after a student wrote a letter to him about mental health.

The boy wanted to showcase the work the school was doing for mental health.

During the Prince of Wales' visit to St Michael's Church of England High School in Rowley Regis he made a joke.

"I'm trying to channel Jack Whitehall, because most of his jokes are pretty dad-like," he said.

"I think what I'll do, is I'll say a joke which is Charlotte's favourite joke at the moment, she keeps telling me.

"Knock, knock," he said, to which the others replied: "Who's there?”

"Interrupting cow, Moo.

"That is what I hear a lot at home at the moment," he said.