China is a rich country, yet food prices are sky high.
The word 'sky' seems an adjective.
Learn English – Meaning of ‘sky high’
meaningnoun-phrases
Related Solutions
"Why is the sky blue?" has the grammatical structure of a question, and cannot be interpreted in any other way.
"Why the sky is blue" has the grammatical structure of a phrase standing in for a noun; it could be replaced by "the reason for the sky's blueness" or "the reason the sky is blue". E.g. one can say "Why the sky is blue is a fascinating question". It can also be interpreted as a question, but technically this is not grammatically correct.
Best Answer
It's an idiomatic way of saying extremely high, exorbitantly high. Sky is not an adjective here; much rather, both words form a single entity that can work as an adjective or an adverb. You will often see it hyphenated, sky-high.