inversion, reverse, turn over, turn around / (computer science) bit inversion / (mathematics) inversion

invert, reverse, turn over, turn around / (computer science) invert bits / (mathematics) invert

thinking; thought process; a train of thought

to think

needed, necessary

need, necessity

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(geology) inversion tectonics

lateral thinking

thinking ability, intellect, mental capacity