Is the distinction between "cup" and "glass" in English clear cut? In Hebrew there is also ספל. The distinction between ספל and כוס is not clear cut. If it's >200 ml, made of glass, and typically holds or is now holding cold beverage - or you're talking about the corresponding amount of the cold beverage itself - it will definitely be כוס (and in English "glass", right?). If it's <100 ml, made of china, and holds or is holding a hot beverage - or you're talking about the corresponding amount of hot beverage - it will definitely be ספל (and in English "cup", right?). Between these two extremes, it's vague in Hebrew. But it follows that כוס has a stronger affinity to "glass" than to "cup".
(However, when specifying quantities in recipes, English uses only "cup", and Hebrew always "כוס".)
a glass, a cup is pronounced kos. with pronunciation kus, it would have a different meaning, which is not in Duolingo. http://context.reverso.net/%D7%AA%D7%A8%D7%92%D7%95%D7%9D/%D7%A2%D7%91%D7%A8%D7%99%D7%AA-%D7%90%D7%A0%D7%92%D7%9C%D7%99%D7%AA/%D7%9B%D7%95%D7%A1#pussy