- A rhetorical question asked to show that the person asking it neither knows the answer nor knows who might.
- It could be one or the other, or both. Who knows?
- Do you think I'll get married before I turn 30? - Who knows, you might never marry.
- A rhetorical question asked to express the idea that anything is possible or that anything could happen.
- Who knows? Maybe someday he'll finish writing his book and make lots of money selling it.
- Since she hasn't studied at all I don't think she'll pass the test, but who knows? She might pull a Homer and actually pass; I doubt it though.