Never forget. 

On the tenth anniversary of the Sept. 11 attacks on the United States, thousands gathered to honor the lives of those lost. Sunday morning Paul Simon performed a heartfelt version of "Sound of Silence" at a memorial held at Ground Zero in New York City.

"Many family members were crying; many others were singing along," NPR's Margot Adler wrote of Simon's performance. It was, "perhaps the most moving moment of the ceremony in New York."

Watch Simon's touching tribute in the video above.