Everybody knows that you yawn when you get tired. But scientists aren't sure what the real cause of yawning is. For a long time, people thought
yawning was caused by the body not getting enough oxygen. However, that turns out not to be true because yawning actually provides less
oxygen than normal breathing does. Some scientists now think that yawning is caused by your brain getting overheated, so you yawn to cool it
back down again. This is a good theory because people's brains do get hotter when they get tired. Another possibility is that yawning may help
to keep people alert, so you automatically yawn whenever your body starts to sense you are sleepy.
Scientists also aren't sure what causes yawns to be "catching" or contagious. Possibly it is an instinctive way for people to communicate to each
other that it is time for all of them to go to sleep. This theory makes sense to us because chimpanzees and monkeys also yawn when they see each
other yawn, and even babies do it too. So maybe it is instinctual behavior. But the weird thing is that sometimes just saying the word "yawn" makes
you yawn too. We yawned a lot while we were writing this report!