What metaphors have you used to convey complex technical terms to the layman or beginner? [closed]

I once used an analogy to explain how RAM is defective to a customer of mine. I started at normal voice level repeating ones and zeroes, then after a few seconds I threw a "TWO!" loudly in there. I then explained that their reaction was how Windows XP/Vista/7 reacts: It freaks out and Bluescreens.

My recent favorite is to imagine their computer is a desk at the library. Once you start opening books, you need to close some to make room to open others. If you leave them all open (I'm talking to you, VP who complains about his "crappy" computer because he leaves 35 windows open at all times) you'll have ripped pages, books falling on the floor, and general chaos.

(oh, did you mean complex topics for us? or just simple concepts like RAM, which are complex to the users?) ;)