十万个为什么·科学阅读30篇（3）
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Why Do We Yawn?
人为什么会打哈欠？
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Yawning opens the mouth wide and pulls in air.
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It may help cool the brain a little.
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When we are tired or bored, brain temperature rises.
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A yawn brings in cooler air from outside.
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It also stretches jaw muscles and increases blood flow.
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Yawning is contagious — seeing it makes us do it too.
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Scientists think this helps groups stay alert together.
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Babies even yawn in the womb before birth.