9/11 was a sad day in the USA.It was when 4 planes got hyjacked and the planes all crash.2 hit the tallest building in New York.The twin towers.The passengers in the first and second plane had know idea what was going on.So that is how to 2 planes crashed into the twin towes to knock the towers down.The  3 plane crashed in the Pentagon.4 plane the plane got hyjacked but the passengers came to save the day.the passengers crash the plane in Pennsulvina so the plane would not hit hit the Washington Monument to kill loads and loads of people.