Here is an astonishing and signficant fact:Mental work alone can't make us tired. It sounds absurd. But a few years ago, scientists tried to find out how long the human could labor without reaching a stage of fatigue . To the amazement of these scientists, they discovered thett blood passing through the brain, when it is active, shows no fatigue at all! If we took a drop of blood from a day labourer, we could find it full of fatigue toxins  and fatigue products. But if we took blood from the brain of Albert Einstein, it would show no fatigue toxing 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 f efforts as at the beginning. The brain is totally tireless. So what makes us tired. Some scientists declare that most of our fatigue come from our mental and emotional  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?Satifaction?No!A feeling of being bored,anger,anxiety,tenseness,worry,a feeling of nt being appreciated---those are emotions that tire sitting workers.Hard work by itself seldom causes fatigue.We get tired because our emotions produce nervousness in the body. According to the authour,which of the following can make sitting worker tired?
Challenge mental work. Unpleasant emotions. Endless tasks. Physical labor.