There are events that are clearly negative that produce memories more vivid than almost any positive memory. I can recall with very great detail where I was when I first heard about the World Trade Center attack in 2001. I can recall many events from that day very specifically. I can recall quite vividly where I was when the shuttle Challenger exploded but I could not for the life of me tell you anything about what I was doing when I learned that the shuttle Columbia burned on reentry. What do you think is different about these negative events that some can recalled so vividly?