Knock-knock!

My 2.5 year old has discovered knock-knock jokes thanks to Yo Gabba Gabba. The original version of the joke went like this:
Knock-knock!
Who's there?
Interrupting cow!
Interrupting co-
MOOO!

Ha ha, right? Then her 9 year old sister invented the Interrupting starfish which involves slapping your hand over the asker's face since (as far as I know) starfish don't make noise. It's hi-larious if you're one of my kids, trust me.

The 2 year old's current version of this joke:

Knock-knock!
Who's there?
Interrupting cow! (or starfish)
Interrupting co-
Why you sad??

Huh? "Why you sad?"?? We have NO idea where this came from. She asks it in the sweetest, most compassionate voice you've ever heard. The contrast between the giggly silliness of the set up and the mature sincerity of her "punchline" never fails to crack me up. It's like the greatest existential joke ever. I can't wait to see what this kid is like when she's older.