Do you know about the Doomsayer? He's the guy dressed in ragged clothes, hanging around on street corners with a cardboard sign that reads, 'The End Is Near!' He's always ready to tell you that mobile phones cause cancer, air traffic fatalities are up this year, and locusts are poised to swarm the city. Having OCD is a lot like having this character living inside your head. He's with you all the time; he knows your deepest fears, and he's not afraid to use them. You try to brush him off at first, but he's so sure of himself, so persistent that you find yourself taking his proclamations to heart. And you worry - a lot. Is my mobile giving me a tumour?What about those locusts? The anxiety of not knowing is unbearable. After a while, you start engaging in little rituals to keep the Doomsayer at bay. You wash your hands three times after you make a phone call. You check the garden once, twice, three times for locusts. You get little relief from your rituals, though. Soon the Doomsayer is onto that trick, and the doubting comes back with a vengeance. What can you do? A lot, it turns out.