Wich event defines "the shot heard around the world"

the Battles of Lexington and Concord and the Siege of Boston, by J. DeCosta July 29, 1775. The first shots were fired just after dawn in Lexington, Massachusetts the morning of the 19th, the "Shot Heard Round the World."

They're referring to the first gun shot at the American Revolution at the Old North Bridge in Concord, Massachusetts, where the first British soldiers dies in the battles of Lexington and Concord :)

four sitting presidents have been assassinated while in office but the first was abraham lincoln,