Yawning is not caused by boredom or tiredness.

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Scientists do not know for sure why humans yawn, but the most likely explanation is that we yawn in order to take in more air. Normal breathing patterns only use our lungs’ lower air sacs, called alveoli – they do not even come close to using our lung capacity! That being said, if these alveoli do not receive air regularly they will collapse a bit and stiffen. Therefore, experts believe that our brains trigger a yawn as a way to keep these sacs flexible.

Other aspects of yawning continue to puzzle scientists, though. No one is quite sure why yawning tends to be “contagious,” and it seems to be a symptom of medical conditions such as multiple sclerosis. Unborn fetuses have also been observed yawning despite the fact that they do not take in oxygen through their lungs. Some male animals, including humans, sometimes yawn in association with penile erection!
(source)

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