I went with "How so?" which I would argue has the same meaning as "How come?" which was another acceptable answer.
Me too. Reported
Similarly, "How's that?"
is there a distinction between 'wieso' and 'warum'? would you use one in some situations and the other in a different context?
Schau mal hier: http://answers.yahoo.com/question/index?qid=20070919145633AANhWMo
Also went with how so, I think it should be correct!