Charlie Veitch was certain that 9/11 was an inside job. The attack on the World Trade Center wasn't the work of Al-Qaeda, but an elaborate conspiracy. He became a darling of so-called "9/11 truthers" - until he actually visited Ground Zero to meet architects, engineers and the relatives of the dead. The trip changed his mind... there was no conspiracy.   His fellow "truthers" did not take Charlie's conversion well.  David McRaney (host of You Are Not So Smart and author of How Minds Change: The Surprising Science of Belief, Opinion and Persuasion) joins Tim Harford to discuss what happened to Charlie Veitch; what it tells us about those who hold strong beliefs even in the face of damning contrary evidence; and why persuasion isn't always the right answer.  See omnystudio.com/listener for privacy information.