We can use a noun as an adjective when it precedes a noun that it modifies; a mountain bike is a bike designed for riding up mountains. 'Mountain' functions as an adjective modifying the noun 'bike'. The second noun takes the plural form, while the first behaves like an adjective and consequently does not, unless the word is normally used in the plural (sports hall) or refers to people (women footballers).
We use these for well-known things, some can be hyphenated and some are written as one word.
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