riddle vs gravel

riddle and gravel both are nouns.

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

As verbs, gravel is a hypernym of riddle; that is, gravel is a word with a broader meaning than riddle:

Other hypernyms of riddle include amaze, baffle, beat, bewilder, dumbfound, flummox, get, mystify, nonplus, perplex, pose, puzzle, stick, stupefy, vex.

riddle (noun)	gravel (noun)
a coarse sieve (as for gravel)	rock fragments and pebbles
a difficult problem

riddle (verb)	gravel (verb)
explain a riddle	be a mystery or bewildering to
set a difficult problem or riddle	cover with gravel
speak in riddles	cause annoyance in; disturb, especially by minor irritations
pierce with many holes
separate with a riddle, as grain from chaff
spread or diffuse through