Hey, Matt:

I work in a convenience store. About half of the transactions are paid for using a credit card. Sometimes the cards are not recognized by the card reader. If this happens, I put a plastic bag over the card and scan it again. Amazingly enough, the card reader now accepts the card. I cannot think of any rational reason why this works. So...why does this work?

-- Steven S. Garretson, Carlsbad

Ah, the old credit-card-wrapped-in-a-plastic-shopping-bag-run-through-a-credit-card-magnetic-strip-scanner-machine trick. Impressive. This is graduate-level convenience-store clerking. The kind of knowledge passed on in the checker game from seasoned veteran to promising rookie. Well, when we're through here, you'll have your digital doctorate. You'll wow all those Slurpee drinkers with your command of the counter. So, grab a Slim Jim and listen up.

Mag-strip thingies are made of magnetizable metal particles embedded in a kind of plastic binder goo. Once it's applied to the back of the card, a very clever machine magnetizes particles in the strip to create the individual pattern of on-off binary bars of the account code for that particular card. When you zip the card through the reader, the on-off mag pattern is translated into current, transmitted to the bank (or whoever) where computers look at the pattern, check the account number against their records, calculate a checksum based on the account numbers, match the checksum against what's registered on the card, and tell you, the clerk, whether everything's copasetic.

Okay. Take a break. Towel off, grab some hot Chee-tos and a Yoo-Hoo, 'cause we're almost done.

Old credit cards are usually the ones that need to be bagged. Through use, the magnetized particles can move out of place or be scratched so badly that they don't translate into the proper electrical signals when the reader scans them. But the misplaced mag particles also have weaker electrical fields around them. When you put the card in a plastic bag, the thin plastic moves the strip far enough away from the machine's strip reader that the weaker signals don't register and the card scans correctly. You can also put a piece of cellophane tape over the strip and achieve the same thing.