十万个为什么·科学启蒙30篇（1）
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Why Do We Yawn When We’re Tired or Bored?
为什么我们在疲倦或无聊时打哈欠？
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Yawning opens your mouth wide and pulls in a large breath of cool air.打哈欠会张大嘴巴，吸入一大口凉气。
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This extra oxygen may help wake up your brain and improve alertness slightly.额外的氧气可能有助于唤醒大脑，略微提升警觉性。
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Brain temperature rises when you're tired, and yawning cools it via blood flow.人疲倦时脑温升高，而打哈欠通过促进血流来降温。
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Contagious yawning happens because mirror neurons fire when we see others yawn.传染性打哈欠源于我们看到他人打哈欠时镜像神经元被激活。
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Children under five rarely catch yawns, suggesting social development matters.五岁以下儿童极少被传染打哈欠，表明这与社会性发展有关。
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Even dogs yawn after watching humans, showing empathy may be involved.狗看到人类打哈欠后也会打哈欠，暗示其中可能存在共情作用。
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Yawning occurs in fetuses before birth, so it’s partly built-in, not learned.胎儿在出生前就会打哈欠，说明这是部分先天行为，而非后天习得。
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It stretches jaw muscles and increases heart rate for short bursts of energy.它能拉伸下颌肌肉，并短暂提高心率以激发能量。
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Some scientists think it prepares the body for transitions—like waking or sleeping.一些科学家认为，它帮助身体适应状态转换——比如清醒或入睡。
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Though common, no single theory fully explains all yawning behavior yet.尽管很常见，目前尚无单一理论能完全解释所有打哈欠行为。