"Nervous exhaustion from mental overwork is most often due to neglect of this rule and the brain worker should limit his regular day's work to a reasonable number of hours per day and those when the brain is at its best."

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Consider a cow. A cow doesn't have the problem-solving skill of a chimpanzee, which has discovered how to get termites out of the ground by putting a stick into a hole. Evolution has developed the brain's ability to solve puzzles, and at the same time has produced in our brain a pleasure of solving problems.

— Martin Gardner