I still don't understand about noun as adjective. can you give me more detail about it?
Such noun-modifiers, however, are not generally classified as true adjectives, since they lack the majority of the syntactic facilities of a typical adjective, e.g. inability to be submodified by an adverb such as 'very' or to form a comparative. (Thus we may not, for instance, speak of *a very rubber ball or of *a more metal racquet.)