I don't know if this is the correct forum to ask this, but I'll give it a try.. It has to do with the phonetic pronounciation of words. Here are two examples: The word "dogs" is pronounced with a [z]-sound at the end. The word "cats" is pronounced with a [s]-sound at the end. Now, my question is this; The choice between the [z] or [s] is determined by a simple phonetic generalisation. All the sounds immediately preceding [z] have one property that differs them from the sounds immediately preceding [s]. What is this property? I know this is a tough nut, but I have an assignment about this due tomorrow and I'm really stuck. Any help would be fantastic.