Do you know that feeling? During take-off and landing you suddenly feel an unpleasant pressure in your ears. They pop and you can't hear very well, sometimes it even hurts. Why is that?
A plane flies at an altitude of up to 12,000 metres (39,370 ft). That is higher than the tallest mountains on earth! The higher it flies, the lower the air pressure outside the plane. To compensate for this, the cabin pressure is increased. We already answered the question how this works in our Children's Question How can you breathe on an aeroplane at an altitude of several thousand metres?.
If the cabin pressure changes, your ears have to adapt to this pressure. That is not quite so simple at such a height. The eardrum in your ear is a strong membrane that "seals" your ear canal off, making it water and airtight. If a plane increases its altitude, for example, the pressure drops. The pressure in your middle ear, however, remains the same and overpressure results in the ear.
The "Eustachian tube" is responsible for the pressure balance in your ear. This is a pipe-like connection between the middle ear and the nasopharynx. It is normally closed, but when you yawn or swallow it opens slightly.
So if you chew something during take-off or landing, or simply yawn heartily, the "Eustachian tube" opens and balances the pressure in your ears. Or, if you hold your nose and firmly press the air to the front. This will also make sure the uncomfortable pressure in your ears vanishes.