There are two reasons aliens would invade Earth.

Either they're friendly scientists trying to do research as was the case in E.T. The Extraterrestrial, Close Encounters of the Third Kind and Starman.
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The problem we have with movies from M. Night Shyamalan is one of expectation. High expectation. For anyone who had the galvanizing experience of viewing The Sixth Sense, it has become impossible not to hope for something special from this filmmaker.

Signs is not that something special, but for at least the first two acts, it's an engrossing film.
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