Sometimes you want Alexa to tell you a knock knock joke, but this is for the times when YOU want to tell HER a knock knock joke!
Just start by saying "Alexa, ask for a knock knock joke", and you know the rest!
Even works with the good old "Banana / Orange" joke, but for "Interrupting Cow" you'll need to add "Alexa" before the "moo", because otherwise she won't listen while she's talking.