An honest man has four stones – white, light gray, dark gray, and black.

He secretly chose one of the stones, and allows you to ask him one question, which he can answer “Yes”, “No”, “I don’t know” or “This is an illegal question”. Any questions assuming other possible answers are not allowed.

Can you determine which stone the man chose?

Disclaimer: It is actually not from the real interview, as the friend of mine gave it to me after hearing my question about honest man and three numbers. I certanly don't recommend asking this one on the interview.

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