Why do my ears pop on a plane?

Because the air pressure in the cabin changes faster than the pressure inside your middle ear can follow, and the pop is the moment the two equalise.

Behind each eardrum is a small air-filled cavity. It connects to the back of your nose and throat by the Eustachian tube, a narrow passage that is normally closed and opens briefly when you swallow, yawn or chew. That tube is the only route by which air can enter or leave the middle ear.

As a plane climbs, cabin pressure falls. The air already trapped behind your eardrum is now at higher pressure than the cabin, so it pushes the eardrum outward until the Eustachian tube opens and releases the excess. On descent the reverse happens: cabin pressure rises faster than air can get in, so the eardrum is pushed inward. Descent is usually the uncomfortable direction, because the tube opens more readily to let air out than to let it in.

The pop is simply the sound of the tube opening and pressure equalising, and the brief muffled feeling beforehand is the eardrum being held out of its neutral position, where it transmits sound less efficiently.

Swallowing, yawning and chewing all work because they activate the muscles that open the tube. Sucking a sweet or sipping a drink works for the same reason. For infants, feeding during descent serves the same purpose.

Congestion from a cold or allergies swells the tube lining and makes equalising much harder, which is when flying becomes genuinely painful. If pain persists after landing, or hearing stays muffled for more than a day or two, that is worth having checked by a doctor rather than waiting out.