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Why do you ask?
Why are you asking?

Do these mean the same and can they be used interchangeably? Thanks.
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whatchadoinDo these mean the same and can they be used interchangeably?
They mean almost exactly the same, but they are not used in the same situations.

You can use both if someone asks you a question. If your friend is planning to ask someone else a question, you might use the second one, but not the first.

CJ
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CalifJimYou can use both if someone asks you a question.
Which is more common when someone asks me a question?
whatchadoinWhich is more common when someone asks me a question?
I think "Why are you asking?" might be more common.

CJ

Why do you ask

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