A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Yawning is commonly associated with tiredness, stress, overwork, lack of stimulation and boredom, though recent studies show it may be linked to the cooling of the brain. In humans, yawning is often triggered by others yawning and is a typical example of positive feedback.
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(thanks Cora)
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