I wasn´t able to explain why the last sentence was so much better, and one student was very frustrated, especially by the idea that the second sentence was better than the first, since it was basically just the first without the apostrophe, reworded.
How can I explain this? What is the grammar rule at work here? I told my student that whenever a phrase like "on top of" could be used, it should be, but that didn´t seem like enough.
Thanks for any help! :)