In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If paper is called wood, wood is called straw, straw is called grass, grass is called rubber and rubber is called cloth, what is the furniture made up of ?
The furniture is made up of 'wood' and as given 'wood' is called 'straw'.
So, the sky is made up of 'straw'.