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.打哈欠时嘴巴张得很大，并吸入空气。
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It may help cool the brain a little.它可能有助于略微降低大脑温度。
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When we are tired or bored, brain temperature rises.当我们疲倦或无聊时，大脑温度会上升。
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A yawn brings in cooler air from outside.打哈欠能吸入外界较凉的空气。
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It also stretches jaw muscles and increases blood flow.它还能拉伸下颌肌肉并增加血流。
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Yawning is contagious — seeing it makes us do it too.打哈欠具有传染性——看到别人打哈欠，我们也会跟着打。
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Scientists think this helps groups stay alert together.科学家认为这有助于群体共同保持警觉。
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Babies even yawn in the womb before birth.胎儿在出生前就在子宫里打哈欠。