It's a familiar scenario for anyone who has ever used plastic to pay: You hand over a perfectly valid credit or debit card at the cash register of your favorite store, but after several swipes at the terminal, it fails to work. (Cue disgusted sigh from the cashier.) 
Is it karmic retribution for buying one too many copies of "People"? No, the most likely culprit is demagnetization, a phenomenon that occurs when the magnetic stripe on the back of the card becomes corrupted. With magnetic stripes on everything from MasterCards to subway tickets -- and magnets built into many everyday objects -- demagnetization can be an accident just waiting to happen. "There are some environments where it's fairly easy to demagnetize a card," says Kevin Rhoads, a research engineer at Dartmouth University in New Hampshire.
But how does magnetic energy result in the ruination of a card? It all has to do with the nature of the magnetic stripe itself. The black stripe on the back is filled with data about your account, from the name of the card issuer to the limits on your card. This data is arranged on the stripe using tiny magnetic particles, so exposure to an external magnet can throw the information out of place and render the card unreadable. "If you disturb the way that the particles were aligned in the first place by putting a magnet close to it, it will disrupt that encoding," says Steve Mathison, vice president of product and business development for First Data, a payment-processing company in Atlanta.
So now that you know how magnets can be the undoing of your card, it's important to know where the most troublesome ones are. Here's a short list of places that you should keep your card far, far away from:
Refrigerator door magnets. No, not the Eiffel Tower-shaped one that you got in France. Rather, it's the magnets that keep your fridge door shut, which are bigger and much stronger than novelty magnets. Bring it within an inch or so of this hotspot, and you might turn your card into a glorified ice-scraper. "If you swipe it past the magnetic door latch, you will wipe it," Rhoads says.
Before you chuckle at the idea of a bank card coming anywhere near the vicinity of a fridge, take the case of this writer, who was using a card to make an online purchase and then hopped up, card in hand, to get a snack. Demagnetization? Accomplished.
Purse and wallet magnets. These magnets' capacity for damage will vary from model to model, but they're usually not strong enough to demagnetize your card through casual, everyday activities like taking your card out of your wallet. Just beware of bringing the card extremely close to the magnet. "If it's over 1 inch away, there's almost certainly no problem," Rhoads says. "But if you put the card stripe directly on the magnetic latch, you can probably erase some of it."
Security tag deactivators. Some stores have small surfaces at checkout stands that a cashier can run a pricey item over to deactivate its security tag. Whether they will demagnetize your card, however, is hit or miss. Some of these deactivators use radio waves to decommission the tag; they pose no threat to cards. Others use magnets, and those can cause damage, Rhoads says. To be safe, put your card back in your wallet promptly after paying.
MRI machines. MRI stands for "magnetic resonance imaging," and with good reason; these pieces of medical equipment use large magnets to create detailed images of things inside the body. The magnetic fields they create are so strong that you shouldn't even bring your cards in the same room. "Don't bring a credit card within yards of one," Rhoads says.
Physical damage. While not a form of demagnetization, mechanical harm to the stripe on your card -- such as scratches -- can render cards useless just as well as magnets. Leaving a card in a car on a hot day can have the same effect. "Anything that causes physical damage to that stripe has the propensity to keep it from being useful in an electronic transaction situation," Mathison says.
In summary: Love your card, treat it well, and it will reward you for years to come (or until it expires).
If the worst happens
Magnetic stripe gone bad? There's no need to walk away from your purchase. There are alternative means of using your credit card if the card reader can't process it, says Mathison from First Data.
The first way is by manual entry -- having the cashier key the 16 or 21 digits on the front of the card into the card reader, along with its expiration date. "The same information will get transferred to the processing company," Mathison says. "It will process as if it were captured by the mag stripe."
If that option isn't available, the cashier can also call the company that processes the store's card transactions and read the card information to a representative, who will then process the payment on the merchant's behalf. A third method is to use a "knuckle-buster" -- an old-fashioned slider machine that takes a carbon-copy imprint of your card for processing later. But don't be surprised if the merchant is unwilling to use this approach. "The problem [with carbon copies] is that I don't know if that card is stolen or over the limit," Mathison says. "I don't even know if that's a real card."
Merchants will usually go through one or more of these steps without prompting because they want your business. "It's rare for a merchant to turn a customer down," Mathison says. "They usually won't say, 'I can't swipe your card, therefore I don't want to do business with you today.'"
See related: Licking, plastic bagging and other credit card secrets, The short, unhappy life of a credit card, Anatomy of a credit card
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