I was wondering how a credit card (without a chip) might be able to store the data needed to make a purchase, so I decided to look it up. As with most things, it actually is related to physics.
I knew that the strip was a "magnet", but in reality, it's several magnets (by several I mean a lot) that are polarized with north and south on differing sides. The strip stores the data you need with these magnets. There are three separate tracks on the magnetic strip that all store different information in almost a sort of magnetic code. When you swipe your card, the card reader decodes your own magnetic "code", allowing you to make a purchase. Neat, right?