WATCH: Prince William's ultimate dad joke
It's a favourite with Princess Charlotte
Prince William proved he is the king of dad jokes as he visited a school in the West Midlands, England earlier today to talk about male mental health – a cause he cares deeply about.
The father-of-three had been asked to come to St. Michael's Church of England High School prepared with his best joke, so he turned to his daughter Princess Charlotte for inspiration and said it was her favourite at the moment.
"That's the greatest one I could think of that's clean and broadcastable," he quipped.
Can you guess which one he chose? We'll give you a clue… It's a classic Knock, Knock joke.
You can find out which one William picked in the video below.