think vs think back

think is a noun but think back is not a noun.

Word	Noun	Adjective	Verb	Adverb
think	Yes	No	Yes	No
think back	No	No	Yes	No

think (noun)	think back (noun)
an instance of deliberate thinking

think (verb)	think back (verb)
bring into a given condition by mental preoccupation	recapture the past; indulge in memories
use or exercise the mind or one's power of reason in order to make inferences, decisions, or arrive at a solution or judgments
judge or regard; look upon; judge
recall knowledge from memory; have a recollection
expect, believe, or suppose
have in mind as a purpose
be capable of conscious thought
focus one's attention on a certain state
decide by pondering, reasoning, or reflecting
ponder; reflect on, or reason about
have or formulate in the mind
dispose the mind in a certain way
imagine or visualize