Whenever the air pressure around you changes, such as when you're flying in an airplane, climbing to or descending from a high altitude, or diving underwater, the pressure in the cavity inside your ear is supposed to change with it. When the pressure change is quite sudden, however, the pressure inside the ear doesn't always catch up right away. This difference in pressure between the ear cavity and the outside environment, called barotrauma, leads to uncomfortable and even painful symptoms, like the following:  Pain or discomfort in the ear A feeling of fullness or pressure in the ear Ringing in the ear (tinnitus) A change in hearing, almost as if you are submerged in water and sounds are muffled In severe cases, hearing loss, bleeding and vomiting To prevent your ear from popping uncomfortably or painfully, you need to stop the difference in pressure from building up.  You can do this by yawning and swallowing, actions that open the Eustachian tubes in your ears, allowing the pressure in your ear to sync up with the pressure in your environment. You can also help yourself swallow by chewing gum, sucking on candy, or nursing a drink, all of which will keep you swallowing. You can do this by performing a simple exercise: close your mouth, pinch your nose, and blow gently. The air has no where to go, so it presses on your Eustachian tubes, which relieves pressure.   Do not blow too hard when you attempt this. If you blow too hard, the maneuver is quite painful, and has the potential to actually damage your ear drums. Blow just hard enough to gently pop your ears. Repeat the maneuver several times, especially during takeoff or descent while traveling by plane. These earplugs are specially designed to gradually equalize the pressure as you gain elevation or descend, so that pressure does not build up in your ears. Filtered earplugs are available at drugstores and in airport shops. While they're not guaranteed to be effective, they can help minimize ear popping while you travel. Barotrauma happens more often when...

TL;DR in Vietnamese:
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