Graham Hess: There is no-one looking out for us. We are all alone.
Ray Reddy: Don't open my pantry, Father. I found one of them in there and I locked him in.
Caroline: Old Mrs. Kendleman twisted her ankle, as she puts it, diving for her life when a bunch of school kids rode down the sidewalk on skateboards. She went down to Thornton's store this morning and started spitting on the new skateboards. Spitting! By the time I got there, Mrs. Kendlemen had sprayed the whole damn place. And she must have had a cold or something. I'm telling you, I won't eat for a week. So, what happened to your crops?
Graham Hess: Morgan, after you were born, the doctor gave you to your mother. When she first looked at you, you just stared right back. You both just stared at each other for longest time, and you didn't even cry.
Merrill: Excluding the possibility that a female Scandinavian Olympian was running around outside our house last night, what else might be a possibility?
Officer Caroline: I'm not done asking questions. And I don't appreciate sarcasm.
Merrill: Move, children. Vamanos.
Graham Hess: We're going to board up every window in this house.
Merrill: How do you know boards will do anything?
Graham Hess: Because they seem to have trouble with pantry doors.
Merrill: Everything has a weakness.
Graham Hess: My vote counts as two.
Morgan: That's bullshit. You're cheating.
Graham Hess: Morgan, calm down.
Morgan: We don't know anything yet.
Merrill: Hey, you guys okay?
Morgan: Some guy had a sign saying it was the end of the world.
Merrill: Don't worry.
Morgan: You won't let anything happen to us, right?
Merrill: No way.
Morgan: I wish you were my dad.
Merrill: What did you say? Don't you ever say anything like that again. Ever.
Graham Hess: I don't want any of you spending time with Tracy Abernathy alone. Understand?
Merrill: It's like War of the Worlds.
Morgan: They said there are one of two outcomes of an invasion. One: they fight, and are defeated, and have to return again with full forces hundreds or even thousands of years later.
Graham Hess: What's two?
Morgan: They win.
Answer: It's left unclear, though the film proposes several possible theories, and there are other possible explanations as well. One major theory brought up in the film is that the aliens were there to acquire Earth's resources after using up their own. Another theory proposed at the end of the movie is that the aliens wanted humans for whatever reason, as a radio DJ mentions seeing his friend's family being dragged away. There is also a very popular fan theory that the aliens are supposed to represent demons that are merely there to create chaos and test the moral strength of humanity, as the film is ultimately about Graham losing and then regaining his faith, although this is more subtext than anything and may not be literally true.