riddle vs stupefy

riddle is a noun but stupefy is not a noun.

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

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

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

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

riddle (verb)	stupefy (verb)
explain a riddle	make senseless or dizzy by or as if by a blow
set a difficult problem or riddle	be a mystery or bewildering to
speak in riddles	make dull or stupid or muddle with drunkenness or infatuation
pierce with many holes
separate with a riddle, as grain from chaff
spread or diffuse through