为什么我们会打哈欠？打哈欠又为什么会传染？
<p>你或许会注意到，聚会的一群人中只要有一个人打了哈欠，其他的人也会接二连三地开始打哈欠，甚至你家的宠物狗狗也会被你的哈欠传染。人为什么会打哈欠，打哈欠又为什么会传染呢？</p><p><span word="You">You</span> <span word="know">know</span> <span word="the">the</span> <span word="feeling">feeling</span>. <span word="It">It</span>’<span word="s">s</span> <span word="impossible">impossible</span> <span word="to">to</span> <span word="resist">resist</span>. <span word="You">You</span> <span word="just">just</span> <span word="need">need</span> <span word="to">to</span> <span word="yawn">yawn</span>.</p><p>你知道这种感觉。那种冲动很难抗拒，你就是想打个哈欠。</p><p><span word="A">A</span> <span word="yawn">yawn</span> <span word="consists">consists</span> <span word="of">of</span> <span word="an">an</span> <span word="extended">extended</span> <span word="gaping">gaping</span> <span word="of">of</span> <span word="the">the</span> <span word="mouth">mouth</span> <span word="followed">followed</span> <span word="by">by</span> <span word="a">a</span> <span word="more">more</span> <span word="rapid">rapid</span> <span word="closure">closure</span>.</p><p>打哈欠时，人们张大嘴巴，之后以更快的速度闭上嘴。</p>
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