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A streamlined body is a shape that lowers the friction drag between a fluid, like air and water, and an object moving through that fluid. Drag is a force that slows down motion; friction drag is a special kind of drag. It occurs when the fluid closest to the object sticks to its surface, exerting a force that opposes the object’s motion. Many animals, such as birds and dolphins, and many machines, such as airplanes and submarines, have streamlined bodies to reduce friction drag as they move through either air or water.