Originally posted by Nuathala
you drop the bottle with ink (eh..or without if it's empty) on the floor 'by accident' then you know for sure that it is empty so you can get a new one.
But that's a waste of ink!
Do you know how expensive invisible ink is?!?
It is very hard to get, you know. Almost everybody spills their ink. I'm not sure of it, but I heared from somebody, who had heared it from a friend, who had heared it from the brother of the wife of the distributer of invisible ink that most people, when they are in doubt wether they have some invisible ink left, 'accidentaly' knock their bottle with ink on the floor, so they are sure they haven't any ink anymore.