Monitor degaussing
When playing with extremely strong magnets, such as those found in batteryless torches, it is wise to not get too near a monitor that you still wish to use. It's true that it makes wonderful rainbow effects, but it's also true that the rainbow stays when the magnet has been removed. This is because the monitor's shadow mask has been permanently magnetised, causing flying electrons to be deflected from where they ought to be hitting.
No problem, I hear you say, just hit the degauss button. Alas, this particular monitor was a dodgy brand that didn't provide such a seemingly obligatory feature. Googling for answers, it seemed the only option was to get hold of a manual degaussing wand, the type that television repairers use.
Obviously, that sounded like too much trouble for me to go to, so in a brainwave I thought of where I could find a "manual" degausser mere metres from the monitor - another, more advanced, monitor. I placed the monitors face-to-face, the rainbow one unplugged, and pressed the manual degauss button. Hey presto, a fixed monitor!