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.
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This extra oxygen may help wake up your brain and improve alertness slightly.
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Brain temperature rises when you're tired, and yawning cools it via blood flow.
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Contagious yawning happens because mirror neurons fire when we see others yawn.
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Children under five rarely catch yawns, suggesting social development matters.
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Even dogs yawn after watching humans, showing empathy may be involved.
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Yawning occurs in fetuses before birth, so it’s partly built-in, not learned.
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It stretches jaw muscles and increases heart rate for short bursts of energy.
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Some scientists think it prepares the body for transitions—like waking or sleeping.
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Though common, no single theory fully explains all yawning behavior yet.