We dive into the science of water security in this week's Naked Scientists. We find out how building a dam alters the local weather, and how simple interventions can help bring safe water and sanitation to the millions that still need it. We find out how new groups set up in Africa and Europe are bringing researchers together to help us use water more efficiently in an ever changing world, and discover the leak-stopping technology that really does hold water. Plus, the secret messages that fish send in ultra-violet and a genetic trick to stop Dengue getting off the ground.