Less than Zero

So I'm looking for some way to strap my boy's DVD player to the back of a car headrest and I come across this on the manufacturer's website. A very simple order form does not require instructions. Indeed, poor instructions could make matters worse.

“Input non-zero quantities for the parts you need” we're advised. Hold on there Einstein! You're talking to a liberal arts major here. I don't live in your cold, cold world of numbers. “Non-zero” you say? Well I did take a little philosophy. I think I can put together a pretty sound argument for why you should money to me when I enter negative numbers. They are, after all, “non-zero.” Maybe I am on my way!