"How come"?

[Guest]

I am a little bit confused about this expression.

Is it just a slang that should not being used in a formal conversation, or can it be used anytime?

To me it sounds like a slang, so i don't dare to use it at all.

[Alan]

This is an expression that is used in conversation and is quite acceptable and direct between people who know each other. It simply means: Why is that?
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English as a Second Language
You can read my ESL story A day in the life of a policeman

[Guest]

Thank you for your answer!