十万个为什么·科学阅读30篇（6）
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Why Do We Yawn When We Are Tired?
人困倦时为什么会打哈欠？
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Yawning opens our mouth wide and pulls in air.打哈欠会张大嘴巴，吸入空气。
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It cools the brain and boosts alertness.它能给大脑降温，提升警觉性。
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When tired, brain temperature rises slightly.人疲倦时，大脑温度会略微升高。
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Deep breaths during yawning bring cool air in.打哈欠时的深呼吸将凉爽空气吸入体内。
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This helps lower brain heat and improve focus.这有助于降低大脑温度，增强专注力。
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Yawning is also contagious among people.打哈欠在人群中还具有传染性。
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Scientists think it may help groups stay in sync.科学家认为，它可能帮助群体保持同步。
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Even babies yawn before birth in the womb.甚至胎儿在出生前就在子宫里打哈欠。