riddle vs penetrate

riddle is a noun but penetrate is not a noun.

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

As verbs, riddle and penetrate are synonyms defined as:

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

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

Other hypernyms of riddle include perforate.

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

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