The Great (with a capital G) Carlo Bergonzi, teaching a young tenor to sing Verdi's difficult Celeste Aïda:
http://www.youtube.com/watch?v=E14YRffugdI
That you are witnessing one of the greatest voice teachers of the 20th century should become instantly obvious to you. But listen to CB's singing at 1:18! It is Gigli reincarnate!