How do we know the time? A clock, a watch or a mobile phone can help us. However, many years ago there were no clocks and knowing the time was not so easy. Over the centuries people have developed different ways of telling the time.

About 5,500 years ago, the Egyptians invented the sun clock. This was a tall stone building. Its shadow(影子)showed the movement of the sun. So people were able to know midday. The Egyptians made a sundial about 3,500 years ago. It was smaller than the sun clock and could measure the time for half a day. On cloudy days or at night it was impossible to tell the time with a sun clock or a sundial. Water clocks were the first clocks not to use the sun. The idea is simple. Water flows from one container to another, When the water reaches a certain level, it moves a lever(控制杆)and this shows the hours. The Egyptians used water clocks about 3,400 years ago. These clocks were popular in the Middle East and China. But they failed to make people know the exact time.

In the 13th century, the mechanical clock was invented. This was more exact, but it was expensive to make one. Over the next few centuries, it was developed. For example, springs(发条)were added around 1,500. This enabled people to get the correct time and allowed clocks to be smaller. In 1936, the first quartz clock was developed. Clocks became cheaper to build. It’s easy for ordinary people to own a clock. People began depending on them more and more to ran business, markets and so on. More recently, in 1956, came the digital clock. And nowadays satellites send our mobile phones the time to the exact second.

There has been a lot of progress in timekeeping. Clocks are always changing but some things never change. Many of us still have trouble getting out of bed on time and not missing appointments.

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Get Stressed, Stay Young

For many years doctors have warned us about the dangers of stress and have given us advice about how to cut down our stress levels. Everyone agrees that long-term stress, for example having to look after someone who has a long-lasting illness, or stressful situations where there is nothing we can do, for example being stuck in a traffic jam, is bad for our health and should be avoided whenever possible. However, some medical experts now believe that certain kinds of stress may actually be good for us.

Dr. Kyriazis, an anti-ageing expert, explains that what he calls “good stress” is beneficial to our health and may, in fact, help us stay young and attractive and even live longer. Dr. Kyriazis says that “good stress” can strengthen our natural defenses which protect us from illnesses common among older people, such as Alzheimers, arthritis, and heart problems. He believes that “good stress” can increase the production of the proteins (蛋白质) that help to repair the body’s cells, including brain cells.

According to Dr. Kyriazis, running for a bus or having to work to a deadline are examples of “good stress”, that is, situations with short-term, low or middle level stress. The stress usually makes us react quickly and efficiently, and gives us a sense of achievement—we did it! However, in both these situations, the stress damages the cells in our body or brain and they start to break down. But then the cells own repair system “turns on” and it produces proteins which repair the damaged cells and remove harmful chemicals that can gradually cause disease. In fact, the body’s reaction is greater than what is needed to repair the damage, so it actually makes the cells stronger than they were before.

“As the body gets older, this self-repair function of the cells starts to slow down,” says Dr. Kyriazis. “The best way to keep the process working efficiently is to ‘exercise’ it, in the same way you would exercise your muscles to keep them strong. This means having a certain amount of stress in our lives.” Other stressful activities that Kyriazis suggests as being “good stress” include redecorating a room in your house over a weekend, packing your suitcase in a hurry to reach the airport on time, shopping for a dinner party during your lunch break or programming your DVD player by following the instruction manual (说明书).

So next time your boss tells you that she wants to see that report finished and on her desk in 45 minutes, don’t worry, just think of it as “good stress” which will have benefits for your long-term health!