penetrate vs riddle

penetrate is not a noun while riddle is a noun.

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

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

As verbs, penetrate and riddle are synonyms defined as:

Other synonyms of penetrate include diffuse, imbue, interpenetrate, permeate, pervade.

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

penetrate (verb)	riddle (verb)
come to understand	explain a riddle
become clear or enter one's consciousness or emotions	set a difficult problem or riddle
pass into or through, often by overcoming resistance	speak in riddles
insert the penis into the vagina or anus of	pierce with many holes
spread or diffuse through	separate with a riddle, as grain from chaff
make one's way deeper into or through	spread or diffuse through
enter a group or organization in order to spy on the members