riddle vs dumbfound

riddle is a noun but dumbfound is not a noun.

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

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

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

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

riddle (verb)	dumbfound (verb)
explain a riddle	be a mystery or bewildering to
set a difficult problem or riddle
speak in riddles
pierce with many holes
separate with a riddle, as grain from chaff
spread or diffuse through