When we are children, the summer holidays seem to last forever, and the wait between Spring Festivals feels like an eternity. But later, we may find that the time just seems to fly by, with weeks, months and entire seasons disappearing from a blurred calendar at a fast speed. Why does time seem to pass faster as we get older?

According to the Daily Mail, our brains degrade as we get older. That diminish the amount of information we can deal with in a single day. “The human mind senses time changing when the perceived images change,” Adrian Bejan from Duke University, US, told the Daily Mail. “The present is different from the past because the mental viewing has changed, not because somebody’s clock rings.”

Infants, for example, move their eyes much more often than adults because they’re processing images at a faster rate. They deal with a large amount of information and do many things in a single day. This makes them feel like a single day lasts for a long time. However, as people get older, fewer images are processed in the same amount of time. Therefore, older people receive less information during a day than younger people. This causes things to seem as though they’re happening more quickly.

Apart from the degradation of our brains, some psychological reasons also make us experience time differently. People may measure time by the number of memorable events that can be recalled within a certain period. When we think about our youth, we may remember a life packed with first-time activities, for example, our first time traveling without our parents, or our first date. We experienced these events so vividly that time then seems to us to have passed very slowly. According to David Eagleman of the Baylor College of Medicine in the US, recalling these memories makes us feel like they took forever. Many adults find life is routine and sometimes dull. For this reason, when they look back, they might feel like there are not many exciting things to remember. Therefore, time seems to be moving faster to them.

In the trailer (拖车式房屋)，Sischo was refreshing the snails ‘(蜗牛）accommodations--an ongoing routine that takes days of careful work. He had found a dozen of Achatinella bulimoides--a third of the world's population of the species. Once every individual was accounted for, he cleaned the cage and packed in new                    leaves. The work took much trouble, but the responsibility, he said, was like “a heavy weight sitting on you.”

The trailer is very vulnerable. It’s designed to keep away would-be thieved, and to resist hurricanes. But a fire could easily destroy it, or a disease could sweep through it. Last September , a mystery pathogon（病原体）appeared to have entered the trailer on leaves fed to the snails, killing almost an entire species. As sad as the event was, there’s no good way to insure against future catastrophe. The snails can’t simply be spread among zoos or other facilities: they need special equipment, experienced handlers, and a diet of native Hawaiian plants.

Consequently, it can be hard for the snails' minders to relax, even when they are outside the trailer. “How do you switch off when your decisions mean existence or extinction?" Sischo said. While action lightens the burden, yet with animals whose natural history is largely unknown, that action can be dangerous. "If you do it wrong, the snails die.”

Snails are neither intelligent nor beloved. Sischo's friends sometimes tease him about being "the strange snail guy' ; strangers ask why he cares. It's hard to convince people, but he insists that if he can just get them in the trailer, they will understand why the Achatinella bulimoides are worth saving. "People melt," he said“When you show them that the entire population is in this chamber, it hits them."

A strong sense of smell is a key component of a healthy and enjoyable life. Yet our sense of smell is in decline as a result of an unnoticed threat to our health: air pollution.

Scientists are finding that anosmia, a loss of the sense of smell, is becoming a widespread problem among people of all ages exposed to PM2. 5 pollutants constantly, which are tiny particles (微粒) that can enter our bodies with every breath we take.

The reason, they suggest, is that the olfactory bulbs (嗅球), which are located in our noses and packed with nerve endings, are affected by exposure to air pollution. The tiny particles cause illness either in the bulbs themselves or in the brain, impacting our sense of smell over time, “Our data show the risk of developing anosmia with constant particulate pollution is 1.6 to 1.7 times higher,” says Ramanathan, a doctor at the Johns Hopkins School of Medicine, America.

Ramanathan is an author of a recent study of nearly 2,700 patients, a fifth of whom had anosmia despite many of them being non- smokers.’ When he and his colleagues looked into the backgrounds of the affected patients, they found most of them lived in neighborhoods with significantly higher levels of air pollution.

The findings prove other studies with similar findings. One of these studies, conducted in a town in Italy, found that more than 200 teenagers and young adults between the ages of 15 and 25 suffered olfactory damage as a result of exposure to NO₂, a common component in traf-fic emissions. “This is alarming as olfactory loss affects patient safety, well-being, and it is a predictor of poor health,” says Ramanathan.

Yet the loss of a sense of smell is a condition that is often overlooked though it can bring about numerous health problems. A sense of smell is linked to memory as well and life is a lot less fun without it. “People don’t remember what the pastry that they ate in childhood looked like, but they remember what the shop smelled like,” says Ramanathan.