sometimes I'll hit 'em with techno-speak like, "well, how much do you know about thermodynamics?<eyes begin to glaze over> you see this right here, thi............hey, where you going?
sometimes I use this line: I dunno, what did you do to it" (if they say I didn't do anything") I say, thats why I'm here
if someone gets real pushy I just say, "well, I could explain it to you, but you probably wouldn't understand what I'm saying and it will add an hour to the bill"
but, I bet nine times outa ten, most people really don't care whats wrong, they just want it fixed
always try and use a sense of humor, it works great and it will get the chump off your back