A wave is a disturbance that propagates (travels) through space and time, usually by transference of energy. A mechanical wave is a wave that propagates through a medium due to restoring forces produced upon its deformation. For example, sound waves propagate via air molecules slamming into their neighbors, which push their neighbors into their neighbors (and so on); when air molecules collide with their neighbors, they also bounce away from them (restoring force). This keeps the molecules from actually traveling with the wave.
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