In everyday street language (as if speaking with a friend):
No way!! How?
"How would there be such a thing?"
Generally, you may say any exclamation, then ask your question, "How?"
That is awesome!! [Informal] How does it work? [Formal or informal]
She knew calculus at age 3?! (“居然可以这样做！is implied and need not be said/verbalized) [Formal or informal]
The following is okay to say in any occasion.
What?? [Immediately followed by] Why?
How did this happen?