All 16 words, sorted alphabetically. Group them into four categories of four.
The four category names, easiest to hardest. No words revealed yet.
Theme name plus one word from each category.
ENAMEL (covers teeth), HAIR (covers scalp), NAIL (covers fingertips), and SKIN (covers the body) are all protective coverings on the human body
CROWD (lost in a crowd), HAYSTACK (needle in a haystack), MILLION (one in a million), and OCEAN (drop in the ocean) all appear in idioms meaning a vast, uncountable mass
COPPER, DICK, FLATFOOT, and GUMSHOE are all old-fashioned slang terms for law enforcement officers or detectives
CAST iron (cast = throw), CHUCK E. Cheese (chuck = throw), HURLY-burly (hurl = throw), and PITCH fork (pitch = throw) all start with synonyms for 'throw'