Nouns sometimes function as adjectives. For example, in each of these phrases, the first word is usually a noun but here functions as an adjective modifying the second word: city government, article writer, bicycle thief, Sunday picnic, pumpkin pie.
When this type of functional switching could cause confusion, consider rewording. Consider this sentence:
Ask the cooler guy if we need more fish.
Here, cooler could be interpreted in two drastically different ways. This alternative phrasing is wordier but clearer:
Ask the guy in charge of the cooler if we need more fish.
Many two-word phrases like the ones listed above eventually become compound words if used often enough—for example, snowman, railroad, toothpick, firefly. But some of these phrases remain two words in standard usage—for example, jet ski, palm tree, fruit fly. There’s no rule governing which become single words and which stay two.
Hyphenated compound nouns
When one noun clearly functions as an adjective modifying another noun, no hyphen is needed. However, when the two nouns are in equal standing, use a hyphen—for example, city-state, poet-novelist, closet-bathroom.