sand clock的用法和樣例：
例句
- Once the grains of sand have fallen through the mitotic hourglass, there's no point in turning it over again.
一旦有絲分裂沙漏里的最后一粒沙落下之后，重頭再來(lái)一遍也就沒(méi)什麼意義了。 - Nothing you or I could do would make more than one grain of sand pass through this narrow neck without impairing the hourglass.
在不損壞沙漏的情況下，不管我們怎么做，都不能讓兩粒沙子同時(shí)通過(guò)狹窄的瓶頸。
