"And you shall write them upon your doorposts and on your gates" - Deuteronomy 6:9.
For an article elaborating on the Torah sources for the Mezuzah's protective powers, click here.
The following is not a real conversation. But it definitely could have happened.
Jeff: Hey bud, what's up?
Mike: Nothin' much, how you doing?
Jeff: Awesome, you gotta come check out my new place on Columbia.
Mike: Sure man, I'd love to.
Three months later...
Mike: Nice place, dude! I like your sculpture there in the corner.
Jeff: Sculpture? I don't have any sculptures in here.
Mike: So what's that big thing there?
Jeff: That's my laundry, bro.
Mike: @#$%.
Mike: Anyway...where's your Mezuzah?
Jeff: Umm..I think it may have gotten stuck in middle of the sculpture..
Mike: Well then, why don't you just call the rabbi and ask him to bring a new one over?
Jeff: He'll really do that?
Mike: Listen man, he isn't a snobby CEO guy, he loves doing this stuff.
Jeff: Wow. Let's do it. How does it work?
Mike: You call/text/email him. He'll bring it over. You give him $40 and it's yours. He'll even give it to you on the spot and you can pay later, or send the bill to mom. She'll be thrilled to get you a Mezuzah!
Jeff: faints out of sheer excitement.