glass 表示玻璃，玻璃杯的时候是单数，但当glasses 意思眼睛的时候，当作复数。
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.
