WHAT?

"Hey you know what?"

The standard reply is What. But Is that a yes or no question? If you replace the initial question with a person's name:

Hey you know Billy?

The standard reply isn't Billy.  It would be yes or no.

However in the rare occurrence of a person's name or nickname being "What" (William Howard Anthony Thompson) then the initial question would be easier understandable as a yes or no question.

One could also take "Hey you know what" as not a question at all but a statement normally followed by a pause.


Contining with our friend "What" one could argue a statement of "What's up" could have a reply of

"Indeed he is, that's cool he got his pilot's license."

If he were outside.  Or how about this statement slightly altered not pertaining to our friend but instead to the actual word "What" making it  "What is up" which could constitute a reply of

"That sucks about the graffitti on the ceiling."

How about an Alternate:

"Hey guess what?"

Sounds more like an order than anything else.  If you're already being told to guess "what" you have no option to guess anything else which negates the guessing in the first place.  Meaning it's not a guess to begin with.