In a world assailed seemingly on all fronts by doomsday scenarios - climate change and global terrorism being merely the latest hot-button topics - are there also things that should make us optimistic about our future? And if there are, what are they and why should they make us look on the bright side? In this, the third collection of answers to the annual www.edge.org question, the world's leading scientists tell us what they are optimistic about, and why. As with its predecessors, What We Believe But Cannot Prove, and What Is Your Dangerous Idea?, the answers provided shed astonishingly varied and revealing light on the most brilliant minds alive today. Global warming getting you down? Lighten up! And read some of the world's brightest brains on their reason to be cheerful