I remember learning this stuff in 1st year at secondary school, and inventing silly turns of phrase like "the pupils' shoelaces" and asking how if you heard it spoken you could tell if it referred to just one pupil's shoelaces or the collective class' combined set of lacework-on-leather pair combinations, which annoyed the teacher no end. While now I'm trying to invent ways of explaining it to my 11-yr-old German daughter, who wrote me a letter in English last week that was, I have to say, pretty literate; if not particularly idiomatic.