So what you have to ask yourself is what kind of person are you. Are you the kind that sees signs, sees miracles? Or do you believe that people just get lucky? Or, look at the question this way. Is it possible that there are no coincidences?
— Graham Hess
A 2002 film, written and directed by M. Night Shyamalan, starring Mel Gibson and Joaquin Phoenix. Weird things start happening all over the globe. Strange sightings of unearthly creatures, crop circles, and other unexplained phenomena start occurring.The story focuses on the Hess family, consisting of a faithless former preacher, Graham (Gibson), his asthmatic son, Morgan (Rory Culkin), his ostensibly autistic daughter, Bo (Abigail Breslin) and his ne'er do well younger brother, Merrill (Phoenix), who live in a podunk farmland community. Like the rest of the world, they start thinking that it's an Alien Invasion being done as a stealth infiltration. The entire story is told through this farming family in Pennsylvania as they watch the paranoia grip the world through their television set. Of course, since an off-screen repulsion isn't too dramatic, there's still a fight scene. Oh, and there's a lot of incredibly bluntreligious symbolism, with the story serving more as a story about a man finding his faith in God again as the "signs" start to coalesce.
This film provides examples of:
Absurdly Ineffective Barricade: At one point, the family barricades all the doors and windows downstairs. Since they don't have enough wood to barricade all the upstairs windows, they instead barricade the doors of every upstairs room which has a window. It's only after the aliens break into the attic that the family realizes they forgot to barricade the attic door.
This is actually lampshaded: when Graham announces they're going to board the windows shut, he's asked how he thinks this will work.
"Because they seem to have trouble with pantry doors."
Artistic License - Biology: The aliens are capable of mastering space travel and being undetectable until planetary approach...yet they lack the intelligence to bring equipment for their "takeover", they seem to have difficulty getting past wooden doors, AND went on a planet that is 71% coated in their number one weakness, ignoring what is present in the atmosphere. This is all important as basic problem-solving is a sign of intelligence, which the aliens SHOULD possess to traverse space but evidently lacks in terms of common sense and strategy 101.
For that matter...what are they bleeding when the water dissolves them? While not all liquids are H20 based, a very large number that can viably exist in a liquid state are.
Why are their bodies harmed by the liquid state of H20 but not what is in the atmosphere? One could justify it in low humidity environments but what about going through clouds, high humidity environments or precipitation?
Artistic License - Chemistry: The film reveals that the aliens can only be destroyed by water, as it seems to dissolve them. The problem is that water is NOT naturally acidic or reactive. Water is actually extremely nonreactive unless either rapidly heated, or in contact with an alkali metal: neither of these conditions are met and even if they were, it would result in a more explosive reaction... NOT AN ACIDIC ONE!
Pure water is pH-neutral, but drinking water from a tap is anything but pure, and is generally slightly towards the acid end of the pH spectrum, as is human skin (and, really, most lifeforms on Earth). The aliens may well be from a planet where everything is shifted toward the alkaline rather than mildly acidic, and so the weak acid of tap water would react exothermically with their skin to form a salt. Though, really, that reaction would still be pretty mild and more likely to cause minor irritation than anything as severe as shown.
Based on a True Story or Based on a Great Big Lie: While the story doesn't cite itself as such, there are a large number of similarities between Signs and the "Hopkinsville Goblins" event in 1955. It's quite possible Shyamalan drew on it for inspiration... whether it's a true story or a hoax is largely immaterial to the movie.
Big Brother Instinct: Morgan has this for Bo. He was even forced to kill Houdini to protect his sister.
Death World: From the aliens point of view; 70% of the planet surface is covered by a fatal, skin-dissolving acidic liquid that also permeates the atmosphere, frequently falling from its skies like rain. All the local flora and fauna are suffused with the acid, with the crowning example being a sentient apex predator that bleeds, spits and excretes the substance through the skin through physical activity. So, uh... WHY did they pick Earth to attack? And why did they not carry protection from this deadly substance?
Demonic Invaders: The viewer who knows his mythology and/or is prone to Fan Wank might note that crop circles, dead livestock, night-time abductions and a fear of water were attributed to fairies, demons and incubi for a long time before aliens found their way into pop culture.
Dull Surprise: Graham and Merrill seem to have...a distinct lack of emotion when Morgan is taken hostage by one of the aliens.
Evil-Detecting Dog: The Hess family dogs go crazy when the aliens are around, but they also become violent and dangerous towards the family. It's also mentioned early on that there have been reports of other animals becoming violent and acting how they'd normally act if a predator was around.
Idiot Ball: The aliens, big time. They walk around without any sort of protection on a planet 70% covered by a substance that kills them but not the people they're antagonising. Why?
There's also the fact that their method of catching humans is apparently the incredibly advanced method of "using their hands." They travel light years across the galaxy, indicating technology far beyond our own, yet all they do is run around trying to kill people like movie boogeymen.
The fact they can't even open a damn door. Who knows, maybe they have become terribly reliant on technological, sliding doors, but all it takes is a basic understanding of levers! Especially given that a wide range of not particularly intelligent Earth animals can work out how to open doors (cats, dogs, etc.)
They also have legs strong enough to leap atop houses, yet they don't even try kicking the door.
Irony: The book on extraterrestrial life the kids buy in town claimed that prospective alien invaders would likely be physically small and weak, as their superior intelligence would make up for it, would use advanced technology to subjugate humanity, and would probably be vegetarians. The actual alien invaders were easily over 7 feet tall, physically strong, and agile (and were obviously not smart enough to realize that attacking a planet and beings made mostly of the one substance that could kill them would be a bad idea), used biological weapons in hand-to-hand combat, and were implied to be carnivores or at least omnivores, based on their muscular builds and the original script suggesting they were harvesting humans for food.
Jump Scare: A few. Examples include the Brazilian news report, Graham seeing an alien on his roof, and the scene where Graham spots an alien's leg sticking out of his crops.
Ludicrous Precision: In a Deleted Scene the aliens attempt to break into the house through the attic door. Merrill stands under it, holding it closed, and on asked by Graham how long he can manage it, replies: "eleven seconds." Thankfully that's long enough for Graham to push a cupboard underneath the door.
Never Trust a Trailer: The films trailers made the film look more like a full out alien invasion/horror movie. While there are certainly elements of that, the actual film is more a story about regaining faith with the aliens being used as a plot device.
No Biochemical Barriers: A rather famous aversion. Water—a very common substance on Earth, which is vital to human survival—is a deadly corrosive chemical to the invading aliens.
Nothing Is Scarier: A glorious example of the third type. Graham is looking at a darkened field with a flashlight. There's nothing there. Then the alien's leg moves.
The classic version is also used in abundance throughout the film. And, Fridge Logic aside, they do a fantastic job.
Too Dumb to Live: The aliens walk around Earth naked, despite being as much vulnerable to water as we are to sulfuric acid. Really, just coming to a planet with a surface area that's roughly three-fourths water probably counts.
To Serve Man: Not explicitly said or shown in the movie, but implied to possibly be the reason the aliens are here. The original script suggested this more strongly.
Water Source Tampering: Bo keeps starting glasses of water only to find something wrong with them (such as "It has [her brother's] amoebas in it!") and stop drinking them, leaving them scattered all over the house, half-full. It turns out leaving the glasses all over was Bo being pre-cognitive but unwilling or unable to explain it. Water is harmful to the hostile aliens, one of which had gotten into the house, and when they need to be able to hurt it, it finds itself standing in a room surrounded by partially full water glasses.