Why do my ears pop on an airplane?

Often when people travel on an airplane their ears will pop, just after take off when the plane is rising and also on the planes decent.

The reason for this is that as we travel away from the earths surface the air becomes less dense. Any air that is trapped in your ear will then cause your eardrums to push outwards. This feeling is the feeling you get just before your ear pops. You will also notice that it becomes harder to hear, so you body then tries to equalize the pressure, by sending the trapped air into your Eustachian tubes. 

The Eustachian tubes are two tubes which connect your inner ears to your throat, this equalization is the sound you hear as a pop!

If on the airplane you find your ear won't pop, try eating sucking the boiled sweet the stewards & stewardess gave you