The fact is that no surface is perfectly smooth. Even an apparently calm pond has little discontinuities on the surface, bits of floating dust, tiny irregularities left over from old waves or something. The interaction of moving air on these surface defects starts a small ripple that offers even more grip on the water by the moving air. As the ripples increase in size the wind adds even more energy and the waves begin to build until the energy loss from the movement of the water matches the energy input from the wind. At this point we may see substantial waves whose size depends on the strength of the wind, its duration and the size of the body of water.
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