e.g. You know what?
(I think this expression is frequently used. What is the meaning of this one? Is it called an echo question? What is the difference between 'You know what?' and 'What do you know?')
No, it's not an echo question.
http://dictionary.reference.com/browse/echo%20question
"You know what?" is usually the introduction to some surprising news.
Ex:
1st person: You know what?
2nd person: No, what?
1st person: My boyfriend gave me a dozen red roses!
"What do you know?" is different. It can be said sarcastically as if in doubt about what the other person just said. It can also be a straight question -- usually followed by something specific -- like -- "What do you know about astronomy?"
" What do you know!" can also be used as an expression of surprise.
As, The soccer team won the state championship. Well, what do you know!
"You know what?" is an expression used to capture someone's attention before sharing something surprising, interesting, or important. It is not an echo question but rather a rhetorical question, meaning that you don't expect a response from the listener.
The phrase "You know what?" is more commonly used to introduce a statement or anecdote, while "What do you know?" is a genuine question asking for information or an opinion. The latter is an open-ended question that invites the listener to share their knowledge or thoughts on a particular subject.
When using "You know what?" you are assuming that the listener is not aware of what you are about to say, whereas "What do you know?" is asking the listener to provide information.