Producer Arif Mardin asked if one of the Bee Gees member
could do some screaming during the main chorus to make the song more exciting.
In response, Barry Gibb began singing higher and higher, eventually singing it
in a falsetto that was unexpectedly powerful. He had never known he had such an
ability and Barry's falsetto became a trademark of the Bee Gees, Gibb recalled
in a May 2001 interview with Mojo magazine "Arif said to me, "Can you
scream?" I said, "under certain circumstances". He said,
"Can you scream in tune?" I said, "well, I'll try"" –wiki