Balls

This is a close-up of one of my son's Christmas presents - it's a "Neocube" which is made up of 216 teeny tiny highly magnetic spheres which you can pull apart and put together in lots of different ways. Some of these involve using a piece of plastic to divide the magnets in a straight line - the idiot child used the nearest piece of plastic which happened to be his debit card, cleverly wiping the magnetic strip and rendering it useless.
Thankfully he wasn't sitting near my credit card at the time otherwise he would have been in a whole world of trouble.