I'd like to talk about the credibility of the aliens. This film leaves much of it to the imagination and focuses on the characters and atmosphere (and since this is an American film, I'm very pleased, coz most Hollywood film makers think ambience is a French perfume and atmosphere is a thing that belongs in a restaurant)and it overall is the same approach as the Brit TV series Dr.Who. As a Dr.Who nut I can use my imagination and with SIGNS you need to use your imagination as you see little of the aliens. I notice the aliens are naked and that their weapon is a gas which sprays from part of their body, so their are naturally deadly like snakes, not creatures who develop technological weapons as we do. Very different. This explains the lack of armor, they probably have thick hide like Alligators (they ARE green!) and the whole thing with water...they are probably hurt by something in Earth's water that is unique to our planet and did not expect this! (Note the Middle East people are the first to discover the "ancient method" of defeating the aliens? This is probably a reference to sprinkling Holy water to ward off demons!)Their purpose is a "raid" to snatch humans swiftly and by stealth, not an all-out invasion. Their ships are certainly advanced, but they may not be anything like we would build, perhaps composed of light or energy or some such to enable them to travel at speeds which metal could not attain. Who knows? And that's the key to this movie, by keeping it enigmatic, the aliens retain credibility if you se your imagination. Why could they not open wooden doors? If they rely on their bodily venomous gas rather than developing "tools" and "weapons" as we have done, then our artificial world would prove a problem for them. They seem to be at remote spots to snatch humans, not trying to stomp cities flat. Overall, I found the aliens in this film well-concieved and very different to the usual Hollywood tentacle-waving types (ala Independence Day). I thought the massive coincidences and proof of everything having a purpose at the end, to restore Gibson's faith, was a bit much to swallow at first, but then again, it's just his way of interpreting what's happened, it's not as if it's being stated as solid fact. All it means is that Gibson has decided to see everything in life as having meaning and purpose again. Overall, a brilliant movie if you use your imagination and think about it, rather than just wanting a typical American SFX fest. The humor was a bit distracting, but it was a great night at the movies and great to rewatch at home!