When computers with CRT's first came into schools, it did not take kids very long to realize they could mess up the display with a magnet. The first time it happened I used my old radio shack demagnetizer to repair the damage. Not too long after that I realized CRT's have degassing coil around the screen that automatically "repairs" damage from magnetic exposure each time the screen it turned on. In the case of severe damage, you have to turn the screen off and on every times to completely eliminate the spots.

Just a bit of useless information as there are so few CRTs around anymore.