什麼情況下glass用單數

glass 表示玻璃，玻璃杯的時候是單數，但當glasses 意思眼睛的時候，當作複數。

例句

A glass of water is on the desk.

There is a glass of water on the desk.

The window glass is shining under the sun.

He always wears glasses.

When the sun shines brightly， people usually wear sun glasses.