Horizon asks some of the biggest names in science to have a quiet word with the new American president. The United States president is quite simply the most powerful man on earth, but past presidents have often known little about science. That is a problem when the decisions they make will affect every one of us, from nuclear proliferation to climate change.To help the new president get to grips with this intimidating responsibility, some of the world's leading scientists, from Dawkins to Watson, share some crucial words of advice.