Here is an astonishing and significant fact: Mental work alone can't make us tire. It sounds absurd. But a years ago, scientists tried to find out how long the human brain could labor without reaching a stage of fatigue (疲劳). To the amazement 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(情绪的) 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 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.

    In our twenties, we find it funny when we can't remember our neighbor's cat's name or a handsome actor who starred in a famous movie. In our thirties, we jokingly call it "brain freeze". In our forties, we laugh it off as a" senior moment" and follow up with one of these old age jokes. But the reality is that there comes a point when being forgetful stops being funny and starts to seem a bit dreadful. You think, "Am I losing it？"Or worse, "Is this a sign of Alzheimer's disease（老年痴呆症）？"

    Well, don't worry. The fact that you recognize your own forgetfulness may be a very good sign, at least in terms of the possibility of your developing Alzheimer's disease（AD）. The researchers in a new study show that it's not forgetfulness but not being aware that we're forgetful that we should fear.

    The researchers began with the assumption that one common feature of Alzheimer's dis-ease is a damaged awareness of illness. And they wanted to prove their theory that the lack of awareness can be used to predict whether someone with" mild cognitive（认知的）damage" will progress to full-on AD. For the study, "mild cognitive damage" was defined as someone whose mental state was considered healthy but who had either complained of memory loss or had suffered objective memory loss.

    The researchers used existing data for 1, 062 people between the ages of 55 and 90 that had been recorded over a 12-year period. The data included brain scans, which the researchers used to look for visual signs of reduced glucose uptake（葡萄糖吸收）.It is an objective marker of the sort of reduced brain function that goes along with AD.As expected, glucose uptake was reduced in those with AD. What the researchers also discovered was that glucose uptake was reduced in those with mild cognitive damage who also showed evidence of reduced illness awareness. Finally, the researchers found that those who had reduced illness awareness were more likely to develop AD than those without.

    In my early thirties, I decided to climb Mount Kilimanjaro, the highest mountain in Africa at 5, 895meters above sea level. 

    After months of preparation, I started my great 1 . At the base of the mountain, I met Mik, a local porter, who warmly 2 me. Mik's job was to carry my equipment up the mountain, set up the tent, and carry everything back down after reaching the 3 . 

    The first day of the climb was 4 the rain forest, across a slippery , muddy ground covered with tree roots and vines (藤蔓) . Mik was carrying 30 kilos on his head! By that evening, we made it to 3, 000meters. The air was 5 and it was colder. When I arrived at the campsite for the night, my tent was already set up and waiting for me. 

    Day two was much steeper and rockier. I really had to 6 what I was doing. And I felt guilty for the tough 7 Mik had to work in. Yet when I turned to Mik, he said with the biggest smile, "pole, pole, "which means"8 , take it easy" in his native language. I 9 back, my burden lighter somehow. 

    Over the next five days, the climb got 10 difficult. The temperatures could 11 from 21℃to below freezing in a few hours. At 5, 700 meters, there's only half the oxygen 12 in each breath compared to at sea level. That leaves many people with severe headache. Yet Mik always had a smile and a positive attitude. It had an enormous impact on mo, giving me the 13 to keep going. 

    Even today, when I find myself 14 with anything in life, I just think back to Mik and his smile. A great attitude can bring joy to those around you, or even 15 strangers to the top of a mountain. 

I was living in Sao Paulo, Brazil. It is a1 city, with more people living in it than in my whole country. We rented a flat in a guarded community, but there was a favela (棚户区) quite 2 .

For several months I didn't have a car, so three times a week, I took buses to go to the city centre. During these 3 , I would ride with the people who took the same bus from the favela to go to work. When I got on the bus, all the seats were already 4 . But when people saw that my bag was heavy, they offered to hold it on their legs to make me feel lighter5 . At first, I was 6 . Then I realized that these people had absolutely no 7 to steal from me: they only wanted to 8 .

Once, on my way back, I had to 9 for a long time at a bus stop. I was alone, except for a woman who was 10 very poor. She carried a small paper bag of popcorn and nothing else.

While we were waiting she walked over and 11 me some popcorn. I 12 her, but didn't want to help myself to it. She then repeatedly insisted that I take from what was clearly her only food.

That was the first time I had thought about how people who have almost nothing are sometimes able to 13 the little they have more "14 "than those who own a lot. I wonder if it's true that the more you have, the bigger the difficulty to share anything with others. I was so moved by that woman's simple generosity that day. I clearly 15 more than she did, but she naturally and joyfully shared what little she had with me.