Do you know why people yawn(打哈欠)? The most obvious answer is that we yawn because we are tired. But why does being tired make us yawn? How does yawning help us? The truth is that we don’t know the answers to these questions. Scientists have different theories about why we yawn, but nobody has been able to prove them.

The first one is the “oxygen theory”. In the past, one of the more popular theories was that we yawn in order to take more oxygen. Of course, when we yawn we breathe more deeply. According to this theory, our brains make us yawn because we need more oxygen. However, these days more and more scientists are against the oxygen theory.

The next one is the “evolution theory.” According to this theory, our ancestors used to show their teeth to dangerous animals. They did that in order to scare the other animals. This theory suggests that modern humans have kept this habit. The problem with this theory is that there isn’t enough evidence to support it.

The most recent theory is the “brain cooling” theory. According to this theory, we yawn when our brains get too warm. Yawning makes our brains cooler again. Why is this important? When our brains are cooler, we can think more clearly. Yawning can help keep us alert(警觉). This theory hasn’t been proven yet, but many scientists think that it will be in the future.

Here are a few other interesting facts about yawning. First, yawning is contagious. If you see another person yawn, then you will be more likely to yawn, too. Second, we yawn even before we are born. Studies have shown that babies yawn while they’re still in their mother’s stomach. They start to yawn after 24 weeks. Finally, we are more likely to yawn when were bored. Why is yawning contagious? Why do we yawn when we’re bored or tired? The truth is that we don’t have any answers to these questions, either.

Most people have a list of wishes—things that they think will bring them happiness. Happiness lists are easy to come up with. However, the mechanism behind them is somewhat complicated, since it involves what psychologist Daniel Gilbert calls the greatest achievement of the human brain—the ability to imagine. To imagine what will bring joy to our future selves requires mental time travel, which is a unique human skill resulting from two million years of evolution. We use this skill every day, predicting our future emotions and then making decisions, whether big or small, according to our forecasts of how they'll make our future selves feel.

Yet, our imagination often fails us. When we're lucky enough to get what we wished for, we discover that it doesn’t come with everlasting happiness. And when the things we feared come to pass, we realize that they don't crush us after all. In dozens of studies, Gilbert has shown that we can mispredict emotional consequences of positive events, such as receiving gifts or winning football games, as much as negative events, like breaking up or losing an election. This impact bias(影响偏差) —overestimation of the intensity and duration of our emotional reactions to future events—is significant, because the prediction of the duration of our future emotions is what often shapes our decisions, including those concerning our happiness.

Just as our immune systems work tirelessly to keep our bodies in good health, our psychological immune systems routinely employ an entire set of cognitive(认知) mechanisms in order to deal with life's habitual attack of less-than-pleasant circumstances. Actually, our psychological immune system has an impressive feature of its own: the ability to produce happiness. Thus, when life disappoints us, we “ignore, transform, and rearrange” information through a variety of creative strategies until the rough edges of negative effects have been dutifully dulled. When we fail to recognize this ability of our psychological immune systems to produce happiness, we're likely to make errors in our affective forecasting.

Happiness, Gilbert points out, is a fast moving target. As passionate as we’re about finding it, we routinely misforecast what will make us happy, and how long our joy will last. In reality, he adds that the best way to make an affective forecast is not to use your imagination, but your eyes. Namely, instead of trying to predict how happy you'll be in a particular future, look closely at those who are already in the future that you’re merely contemplating(冥想) and ask how happy they are. If something makes others happy, it'll likely make you happy as well.

Those short metal fences by the side of the road don't look like much. But they do a big job: keep drivers safe.

People who design highways would always like to have a wide,flat,clear space on either side of the road. That way if a car goes off the road, it can stop safely.

But in some places, that is just not possible. In that case, they put up a guardrail. These short railings are designed to stop cars from running into something more dangerous,without damaging the car too much.

Jersey barriers are made of concrete.They are ofen placed down the center of a road to separate cars going into opposite directions.Jersey barriers are also used around construction zones. They are cheap to make and easy to move around.

A strong-post W-beam guardrail,with short,thick posts set together, is used where it's important to stop a car quickly to save lives,like being near a cliff. It gets its name from the fact that if it's cut in half, the rail looks like a sideways W.A weak-post W-beam guardrail has posts thinner and farther apart.And it will stop a car more slowly than a strong-post guardrail.The more gradually a car slows down, the less likely its riders will be hurt. A weak-post guardrail can be used where there is more space to slow a car down.

A three-beam guardrail has a piece of steel that ripples (呈波形） in three C-shapes.This type of guardrail is used on roads with a lot of truck traffic. The curves in the C-shapes absorb energy in a crash, and the rail is high off the ground to catch trucks.