Can we use do-auxillity verb in indirection question sentence. 

For example, Is this right?

Why is the sky blue?-> Why the sky do blue? Or Why the water do exchange to air on 100 degree?



Why is the sky blue?

The indicative version would be

The sky is blue.

..here, the sky is ascribed a property ("blue"). "Blue" is not a verb, it's an adjective. If you write

Why does the sky blue? (WRONG)

.. you'll make it seem as if it's a verb. "The sky started to blue". But there is no single-word verb meaning "to blue" in English.

The proper way would be to say:

Why is the sky blue?


Why does the sky look blue? (as Catija wrote).

With do, you need a verb like look, not an adjective like blue.

In your second sentence, you have a verb ("turn to steam"), so you can use the so-called "do-support":

Why does water turn to steam at 100C?

Related reading:


In your specific examples, you can say:

Why does the sky look blue?

Why does water turn to steam [or evaporate/vaporize] at 100 degrees C. -This statement actually wouldn't be correct with is.

Do generally can't be used to replace is.

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