The dark stripe on the back side of the credit card is made up of a bunch of tiny magnetic particles bound in plastic. The particles are arranged in magnetic and non-magnetic “zones” to encode the data, like your account number, expiration date, etc., that the card reader needs to process the transaction. When you swipe the card, the card reader reads the information by detecting the changes between the zones.
The strip is delicate, and the data on it can be corrupted by
exposing it to a strong magnet or scratching it. Some of the
magnetic particles can get dragged out of position. If enough
magnetic bits move into a non-magnetic space to create a weak
signal, the data gets corrupted and the card reader gets an error.
Applying Scotch tape to the magnetic stripe, encasing the card in a
plastic baggie, rubbing the card on clothing, or wrapping the
plastic in a dollar bill or a register receipt may enable a cashier
to complete the transaction. Also, licking the mag stripe, applying
and removing Scotch tape, or rubbing it on your clothes can remove
dirt and debris that may be preventing the reader from accepting the
When the cashier puts the card in a plastic bag, it creates a spacer
so the card slides through the reader with a slight separation
between the data stripe and the stripe-reading head. The separation
weakens the signal and cleans it up. With just a little bit of
magnetic material in them, the contaminated non-magnetic zones still
have a much lower magnetic strength than the parts that are supposed
to magnetized. Increasing the distance between the card reader and
the corrupted zones is enough to get the reader to read those weak
parts as non-magnetized again.