Thinking (n.) is a potentially dangerous practice in that it cannot be stopped once it is learned... so much so that silence makes the thinker extremely uncomfortable.
Thinking (n.) is, when done in excess, to have nothing to think about except thoughts. It is to lose touch with reality and live in a world of illusion.
Thinking (n.) must stop at some point, like boiling an egg — a fact philosophers find hard to recognize.
Thinking (n.) is a social activity, a mutual exchange of messages resting on languages, libraries and museums; an intelligent person implies an intelligent society.
Thinking (n.) that feels effortful is probably a disorder, as a heart we can feel pounding is out of order: the brain is not a muscle.
Thinking (n.) is silent talking, and its only novelties are rearrangements of old words and ideas.