The expression "guess what" is an enthusiastic one, it means something like: "Can you guess what I am thinking?" or "Can you guess an interesting thing I am thinking about right now?"
The expression "you know what?" is similar to the previous one, it means something like "Do you know what I am thinking?" or "Do you know what I am going to talk about?"
Of course although both expressions have a question structure, the speaker does not expect them to be answered. In fact the speaker never expects his interlocutor to properly guess or know what he is thinking. This expression is just used as a kind of "link" to connect ideas. See the following examples:
- "Emily's English is becoming better and better."
- "Yes, and you know what, I think she is planning to travel to London next year."
- "Are you sure? Guess what? I think I'll do the same!"
How would you use these expressions in the "Outer Space Language" ?
PS Not a native speaker