A '''sail''' is a surface intended to generate thrust by being placed in a wind. Depending on the angle between the wind and the surface, one side of the sail will have a higher air pressure than the other one. This difference will cause the sail to pull towards the area of lower pressure.
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Sails are primarily used at sea, on sailing ships as a locomotive system, but have been rendered commercially obsolete by other forms of propulsion, such as the steam engine. For recreation, however, sailing vehicles remain popular.