Tuning your ear

Years ago I trained to be a piano tuner.  In the beginning, I could not hear what I needed to hear..  But i still had to practice.  Then after hours and hours of listening..  I finally "thought" I heard something.  I kept listening for hours.. Then I would know... that I heard something.  But even then it would only last for a few seconds and then go away.  After constant practice and listening.. I got pretty good at hearing  the sounds that I needed to hear in order to tune a piano.