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Phrase(s): How come?
How did that come about?; Why? • Sally: I have to go to the doctor. Mary: How come? Sally: I’m sick, silly. • John: I have to leave now. Bill: How come? John: I just have to, that’s all. • Henry: How come you always put your right shoe on first? Rachel: Do I have to have a reason for something like that?
McGrawhill's American Idioms And Phrasal Verbs