Rain,snow,ice,wind, and plant roots rubbed away at the top of the new canyon.

VOA: special.2009.10.07

So a closed system would be, for instance, a glass of ice water with an ice cube in it, with the lid on top.

因此闭系可以是，比如说，一杯放了,一个冰块的冰水，顶上加了盖子。

麻省理工公开课 - 热力学与动力学课程节选