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The jaw joint sits extremely close to structures of the ear. That's one reason chewing, yawning, or moving the jaw can sometimes change sensations of pressure around the ears. Yawning can also help open the Eustachian tubes, which equalize pressure between the middle ear and the surrounding environment.
This is why people instinctively yawn or chew gum during altitude changes. Two body systems that seem unrelated are positioned surprisingly close together. The mouth can indirectly help the ears adjust.
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