Graham Hess: Swing away Merrill. Merrill... swing away.
Graham Hess: Everybody in this family needs to just calm down and eat some fruit or something.
Bo: There's a monster outside my room, can I have a glass of water?
Merrill: Move children! Vámanos.
Graham Hess: It's happening.
Morgan: I think God did it.
Merrill: Move, children. Vamanos.
Officer Caroline: Merrill... how's work at the gas station?
Merrill: Stimulating.
Graham Hess: What are you thinking about?
Bo: Why do you talk to Mom when you're by yourself?
Graham Hess: Makes me feel better.
Bo: Does she ever answer back?
Graham Hess: No.
Bo: She never answers me either.
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.
Merrill: There are a lot of things I can take, and some things I can't. But what I can't take is when my older brother, who's everything that I want to be, starts losing faith in things. I saw that look in your eyes last night. I don't ever want to see that look in your eyes again.
Graham Hess: You're scaring your sister.
Bo: I'm already scared.
Colleen: Tell Morgan to play games, it's okay to be silly.
Graham Hess: I will.
Colleen: Tell Bo to listen to her brother. He'll always take care of her.
Graham Hess: I will.
Colleen: And tell Graham.
Graham Hess: I'm here.
Colleen: Tell Graham... see. Tell him to see. And tell Merrill to swing away.
Merrill: The nerds were right.
Morgan: It's not contaminated, you don't even know what that word means.
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.
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.
Merrill: Everything has a weakness.
Graham Hess: I don't want any of you spending time with Tracy Abernathy alone. Understand?
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.