of these scientists，they discovered that blood passing through the brain，when it is active，shows no fatigue at all！If we took a drop of blood from a day laborer，we would find it full of fatigue toxins(毒素) and fatigue products.But if we took blood from the brain of an Albert Einstein，it would show no fatigue toxins at the end of the day.

　　So far as the brain is concerned，it can work as well and swiftly at the end of eight or even twelve hours of effort as at the beginning.The brain is totally tireless.So what makes us tired?

　　Some scientists declare that most of our fatigue comes from our mental and emotional(情感的) attitudes.One of England’s most outstanding scientists，J.A.Hadfield，says，“The greater part of the fatigue from which we suffer is of mental origin.In fact，fatigue of purely physical origin is rare.” Dr.Brill，a famous American scientist，goes even further.He declares，“One hundred percent of the fatigue of a sitting worker in good health is due to emotional problems.”

　　What kinds of emotions make sitting workers tired？Joy？Satisfaction？No！A feeling of being bored，anger，anxiety，tenseness，worry，a feeling of not being appreciated—those are the emotions that tire sitting workers.Hard work by itself seldom causes fatigue.We get tired because our emotions produce nervousness in the body.

　　(2017·辽宁，C)

　　5．What surprised the scientists a few years ago?

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