Reverse knock knock joke

Told to be by my dad about 20 years ago:

Dad: Got a good joke for you, it's a knock knock joke

Me: OK...

Dad: you have to start...

Me: OK, Knock knock

Dad: Who's there.....

Me: ಠ_ಠ .... Well I don't know, IT'S YOUR JOKE!

(Dad in stitches)