Yawning sends out certain messages -either "Oh,this movie is so boring" or "I probably need to get some sleep". But did you know that a yawn can also help your brain to cool down when it is overheated? A new study, led by a research team at Princeton University, has indicated that yawning could be the brain's natural way of regulating temperature. People yawn more often when the temperature outdoors is lower than their body temperature but are less likely to yawn when it is hotter outdoors, according to Sciencedaily. com. The research team did an experiment on 160 people, 80 in summer and 80 in winter, to examine how often they yawned at different air temperatures. The study found that people yawned more often in winter than in summer. Scientists say that when the air temperature is lower outside the body, there is heat exchange between the overheated brain and the cool air. But when the air temperature is higher than or equal to the body, people are less likely to yawn because the hot air they breathe in will make the brain even hotter. When people yawn, their jaw is also stretched,which increases blood flow and may also help cool the brain The study showed that the amount someone yawned could be related to the amount of time they spent outside. The longer they spent outside in summer, the less they yawned. Nearly 40 percent of participants yawned within their first five minutes outside, but after that the percentage was reduced to less than 10 percent. However, the result was the opposite in winter. The number of people who yawned increased when they spent more than five minutes outdoors. But the change was only slight compared to summertime. According to Gallup, this is the first report to show that yawning frequencies change depending on season. This could help us to understand better the way our brains work. It may also help us understand the reason why frequent yawning can sometimes be a sign of brain disease. Acorrding to the text,people yawn more often   _  .
when the air temperature is equal to the body when the hot air breathed in makes the brain hotter when the air temperature is higher than the body when the air temperature is lower outside the body