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I think that most of them are common or not so common aerial phenomena which the viewer misinterprets because of unfamiliarity or simply because deep down he “wants” to see an alien spacecraft.  For example, Kenneth Arnold’s original description of his famous sighting — a formation of crescent shaped metallic objects (not saucers), which seemed to be skimming through the air like stones skipped over a pond.  Any birdwatcher instantly recognizes a description of a flock of large birds flying at just the right distance that you can’t quiet make them out, but the movement of the wings up and down that gives you that almost characteristic skimming motion.  I think most UFOs boil down to something like that, if only they could be investigated thoroughly.