That was a joke. Let me tell you the reason why it is funny...
What is Harry Potters favorite method of getting down a hill?
What is Harry Potters favorite method of getting down a hill? Walking... JK, Rolling.
To be funny, this joke requires the reader to know that the penname of the author of the Harry Potter books is J.K. Rowling, and also to recognize the text speech abbreviation JK (which stands for just kidding). Even then it is not really that funny. The humor lies in the fact that the last sentence in the joke (JK Rolling), has two meanings. It could mean that the jokester was kidding, and that the correct answer is rolling, or it could just be an attribution of the prior statement to J.K. Rowling. This joke, therefore, is a pun.