You know that intro bit to Mission Impossible series: “this message will self-destruct in 5 seconds” or how ever long it was?  There is now promise of a self-erasing ink that will do just that in a pre-programmed amount of time.  Less dramatic than a disk bursting into flame but just as effective.  It’s obvious why people would get excited about secret government documents and expiring train tickets and passes but what about all the other uses?  High School love notes that would no longer be evidence by the time class was over, etc. Of course,that would be driving us back to a paper-based society (which we don’t seem to have ever left) since if you post the same material to Facebook it’s kinda a mute point.