On this date, four passenger planes in America were hijacked. The men on these planes carried knives, and they took over flying the planes. They flew two of them into tall buildings, and one into the Pentagon, an important government building.

The fourth plane crashed in a field after the passengers on board fought the hijackers.

Many people were killed, and since then, airports around the world have been a lot more careful about security, and checking what passengers carry on board planes.