solve vs riddle

solve is not a noun while riddle is a noun.

Word	Noun	Adjective	Verb	Adverb
solve	No	No	Yes	No
riddle	Yes	No	Yes	No

As verbs, riddle is a hyponym of solve; that is, riddle is a word with a more specific, narrower meaning than solve:

solve: find the solution to (a problem or question) or understand the meaning of
riddle: explain a riddle

Other hyponyms of solve include answer, resolve, strike, guess, infer, answer, break.

solve (noun)	riddle (noun)
	a coarse sieve (as for gravel)
	a difficult problem

solve (verb)	riddle (verb)
find the solution to (a problem or question) or understand the meaning of	explain a riddle
find the solution	set a difficult problem or riddle
settle, as of a debt	speak in riddles
	pierce with many holes
	separate with a riddle, as grain from chaff
	spread or diffuse through