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Yawning is an involuntary action that causes us to open our mouth wide and breathe in deeply. We yawn when oxygen levels in our lungs become low. Studies show that during normal breathing we don't use anywhere near lung capacity. The air sacs or alveoli needs fresh air or else they tend to stiffen up a bit. 

Therefore it is believed that the brain prompts the body to either sigh or take a yawn to get more air into the lungs.