thought-provoking vs challenging

thought-provoking and challenging both are adjectives.

Word	Noun	Adjective	Verb	Adverb
thought-provoking	No	Yes	No	No
challenging	No	Yes	No	No

As adjectives, thought-provoking and challenging are synonyms defined as:

thought-provoking (adjective)	challenging (adjective)
stimulating interest or thought	requiring full use of your abilities or resources
	disturbingly provocative
	stimulating interest or thought