In theory, sailboats should only be able to go one direction: the way the wind blows. But modern boats and sails take advantage of the same laws of aerodynamics that allow airplanes to fly. No boat can sail directly upwind, but most can come within 45 degrees of doing that. To reach a destination upwind, a boat sails a zigzag course. If the sailboat maintains the right attitude to the wind, the very thing that should have pushed it backward propels it forward.