While his parents were away, a teenage boy and his friends drank some of the parents' gin. This, of course, was strictly forbidden. They then poured water into the gin bottle to return the level to where it had originally been and put the bottle back exactly where they had found it. However, when the couple came home, the father took one look at the bottle of gin and turned angrily to his son to denounce him for illicit drinking. How had he known?