The video: On Sunday, at New York City's ceremony marking the 10th anniversary of the Sept. 11 attacks, singer-songwriter Paul Simon was supposed to sing "Bridge Over Troubled Water." Instead, Simon surprised the thousands of mourners and dignitaries at Ground Zero — President Obama and former President George W. Bush were both in attendance — with a more brooding choice: "The Sound of Silence." (Watch the performance below.) The song — which opens with the lyric "Hello darkness, my old friend" — was originally written in the wake of the JFK assassination. On Sunday, as Simon emotively sang, accompanying himself on guitar, many Americans at Ground Zero — and perhaps across the country — broke down in tears.

The reaction: Simon's performance was "heartbreaking" and "emotionally wrenching," says People. Yes, there are many songs associated with 9/11, says Mack Rawden at Pop Blend. Simon himself performed two of them, "The Boxer" and "Bridge Over Troubled Water," in the days after the attacks. But Sunday's performance "topped them all." Never before "have his words seemed so moving." Indeed, says Max Read at Gawker. Going off-program was a great call. "The lyrics to 'Sound of Silence' are much better, and arguably more appropriate," than "Bridge Over Troubled Water" — and their meaning has clearly "changed between 1964 and 2001, and between 2001 and now." Watch for yourself: