What do you know?
Inf. a typical inquiry on greeting someone. (A specific answer is not expected. Often pronounced "Wha-da-ya know?") Bob: Hey, Tom! What do you know? Tom: Look who's here! Hi, Bob! John: What do you know? Mary: Nothing. How are you? John: Okay.
What do you know (about that)?
Inf. That is very interesting. Tom: I heard that Jim and Mary are getting married. Jane: Well! What do you know about that? What do you know? Bill finally sold his house!