What did the government have to do to stop other 9/11 attacks

They made security more secure at airports making it harder to sneak weapons in and impossible to get any type of gun,knife, or bomb into the airport and especially a plane. Even common everyday items are restricted on airplanes depending on how big it is and what it weighs.

There are two people that I can think of, Signing Freud and Wilhelm Wundt. I believe Freud is the greatest of all time though.