WHY is Simon Cowell smiling?

Because it’s the moment when he discovered Susan Boyle, who makes her American performance debut during tomorrow night’s finale of “America’s Got Talent.”

Piers Morgan wrote about this very grin in UK’s Daily Mail earlier this month. Invited over to Cowell’s $2.5 million Los Angeles abode, Morgan recalls a Saturday spent watching Boyle’s original audition clip in Cowell’s private cinema.

“After 90 seconds, we got to the close-up, where Simon suddenly realizes Susan’s going to make him a fortune,” Piers wrote. ” ‘I love that bit,’ [Cowell] said. ‘It has to be the best smile in TV history. . .’ ”