Echoes

Tom, a friend of mine was recently on holidays with his family in Swiss Alps.
One day, he went for a walk through a deep valley. He decided to its reputation for amazing echoes.
"HELLO!" he shouted out and, sure enough, back came the reply, "HELLO! Hello! hello!"
Very much amused by this, he shouted out "TOM!"
Much to his suprise, back came another reply. "YES!"