Every person yawns. So do many other vertebrate （有脊椎的） animals. including snakes. Dogs, cats, sharks, and chimpanzees. While yawning is contagious （会传染的）. not everyone catches a yawn. Around 60-70% of people yawn if they see another person yawn in real life or in a photo or even read about yawning. Contagious yawning also occurs in animals, but it doesn't necessarily work the same way as in people.

Scientists have proposed many theories for why we catch yawns. Probably the most popular theory is that yawning serves as a form of nonverbal communication. Catching a yawn shows you're accustomed to a person's emotions. Scientific evidence comes from a 2010 study at the University of Connecticut. which concluded yawning does not become contagious until a child is about four years old. when empathy （共情） skills develop. In the study, children with autism, who may have impaired empathy development, caught yawns less often than their peers.

Studying contagious yawning in other animals may provide clues to how people catch yawns. Contagious yawning in animals may serve as a means of communication. Siamese fighting fish yawn when they see their mirror image or another fighting fish, generally just prior to an attack.

Contagious yawning is linked to temperature, in both animals and people. Most scientists speculate it is a thermoregulatory behavior, while some researchers believe it is used to communicate a potential threat or stressful situation. A 2010 study of budgerigars （虎皮鹦鹉） found that yawning increased as temperature was raised near body temperature.

People commonly yawn when tired or bored. Similar behavior is seen in animals. One study found the brain temperature in sleep deprived rats was higher than their core temperature. Yawning reduced brain temperature, possibly improving brain function. Researchers will do further study on this.

A study of 8 different experiments showed that our brains tend to prefer addition rather than subtraction when it comes to finding solutions—in many cases, it seems we just don't consider the strategy of taking something away at all.

The researchers found that this preference for adding was noticeable in three situations in particular: when people were under higher cognitive (认知的) load, when there was less time to consider the other options, and when volunteers didn't get a specific reminder that subtracting was an option. In one of the experiments, participants were asked to improve a Lego structure so that it was able to take more weight. Half the volunteers were reminded that they could take away bricks as well as add them, and half weren't. In the group that got the reminder, 61 percent solved the problem by taking away a brick—which was a much faster and more efficient way of making the structure stable. In the group that didn't get the reminder, only 41 percent went for the removing bricks approach.

"Additive ideas come to mind quickly and easily, but subtractive ideas require more cognitive effort," says psychologist Benjamin Converse, from the University of Virginia. "Because people are often moving fast and working with the first ideas that come to mind, they end up accepting additive solutions without considering subtraction at all."

The researchers have a few ideas about what might be going on. Our brains might find additive changes easier to process perhaps, or we might be associating adding with ideas of something that's bigger and therefore better in our subconscious. There might also be associations in our minds with the current status being something that needs to be maintained as much as possible—and taking something away is arguably more destructive to the current status than adding something new.

The researchers say their work is important in a much broader sense: for institutions looking to streamline (简化), for example, and even for the human race looking for ways to better manage the planet's resources.

Though it's common that new scientists get international experience as part of their postdoctoral work, acquiring international exposure during one's PhD study is less common. But it can be rewarding.

I'm in the final year of my PhD course in experimental physics. The PhD program is based at Loughborough University, the UK, but a big part of my experimental work is at the Max Planck Institute in Germany. Coming from England just after my bachelor's degree in physics was a big step and somewhat risky. I wasn't sure if it would help my career or suit my personal style. I was faced with a number of questions.

My supervisors（导师）, one in England and one in Germany, gave me great freedom to pick research topics and carry out my research at another institute.

I was excited about going to the Max Planck Institute. Once the long road of the PhD is complete, I hope to have the confidence to compete in an increasingly multinational field in order to secure postdoctoral positions, Although moving abroad means reduced contact with my home university and research community, attending conferences in Britain has helped me network and develop a sense of community.

It took some time to settle down, find an apartment, and get used to the local life, surroundings and a different language. In the lab, a simple task was complicated by the language barrier. Still, with persistence, it was an enriching experience both personally and professionally. The Max Planck Institute provided not only an exciting environment, but enough funding that helped expand the output of my research considerably. I had access to equipment and resources hard to find at many institutions.

Thus far, I have no regrets. Doing a PhD abroad has improved my knowledge of physics and clarified my career aspirations（抱负）.