Keith Urban got visibly emotional when Kelly Clarkson returned to the stage of American Idol this week, and in a new interview, he explains his reaction.

Clarkson shot to fame after winning the first season of American Idol, and she's just one of the alumni who are returning to the show in its final season to say farewell to what has become an American television institution. She gave a searingly powerful performance of "Piece by Piece," the title song of her current album, and could barely get through it in some spots, she was so emotional. That emotion translated to the fans and even the judges, with Urban tearing up and openly weeping watching Clarkson sing.

The song talks about how her husband is the father that her own father never was, and it seemed to especially resonate with Urban, who is a father himself.

"This song is insane," he tells Entertainment Tonight. "I mean, I figure you're either crying at this performance or you need to check your pulse. It's an amazing song."

Clarkson is pregnant with her second child, and she tried to blame her emotions on hormones. Urban joked that his reaction might have been caused by the same thing. "I was wondering if I was, too! Tears were just falling out. But it was such a ...  I mean, separate to her incredible gift as a singer, and the song and the performance, the humanity in that moment was just ... you never see that, let alone on television. I was so grateful to be there to witness it."

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