Training your ear is much the same as training your dog: you keep shouting orders at it until it recognizes what a certain sound means.

For example, if someone sat at your piano and kept playing major chords without interruption for two straight days, you would no doubt recognize a major chord any time one sounded during the remainder of your life!

Or you might have the same dedicated friend strike nothing but major thirds: C and E, D and F#, F and A, etc. And the next time you heard your car horn you would probably exclaim,