glass作“玻璃”时，是不可数名词；作“玻璃杯、一玻璃杯的量”时，是可数名词；glass的复数形式glasses有“眼镜”意，此时只用作复数。
The water was as smooth as glass.
水平如镜。
A glass of mineral water, please.
请给我来杯矿泉水。
The city is characterized by tall modern buildings in steel and glass.
这座城市的`特点是钢铁和玻璃建造的现代化高楼大厦林立。
