how would I program a knock-knock joke?

I'm trying to make Siri ask me a question based on the last thing I said. I'd like to start with a Knock Knock joke.

That is - if I say "HEY SIRI! Knock-Knock..." and Siri says "WHO'S THERE?"

Then I want to be able to tell it several different knock-knock jokes, depending on my first "who's there".

I'm guessing i should use an "IF" statement, but I'm darned if I can figure out [anything] about short cuts.

Is there a tutorial geared toward a mentally challenged luddite like me?

Thanks for whatever help you can offer! :)