Dear Matthew Alice: Every time I’m in the grocery store, I always wonder how the machine that reads the UPC bar codes can tell what is front and what is back. They don’t line up the boxes in any special way before they pull them across the machine, so how does it know if it’s reading the number backwards or forwards? This may seem like a dumb question, but I’ve always wondered. — Anonymous, Somewhere in Faxville

Remember our motto, Anon, there are no dumb questions, just...um.... No, at our last meeting I think we decided that wasn’t a very good motto. We’ll come up with a new one. Anyway, bar codes on grocery products are divided into two sections, the five-digit manufacturer number and the five-digit product number. Plus some other stuff we’ll ignore for now. Each digit is represented in the bar code by a pattern of 1s and 0s (the width of each black bar and each white space tells the code reader whether it’s a 1 or a 0). The manufacturer number bar code system has an odd number of Is in the pattern for each digit; the system for the product code reverses this, having an even number of Is in each digit pattern. That’s how the code reader knows which is which. Come to think of it, our new motto might be, There are no dumb questions, just answers that are so hard to explain they give me a headache.