Ancient cultures broke off pieces of obsidian to make knives and weapons such as arrows.

VOA: special.2009.09.16

You just kind of swept them under the rug, but now, you are out there in full force with your knives sharpened.

只是把它们隐藏起来了，但是现在，竭尽全力站在那儿,拿着削尖了刀。

麻省理工公开课 - 固态化学导论课程节选

Who could forget the ghostly figure of Freddy Krueger: a disfigured face long razor-sharp knives flash from his fingers and his sinister way of turning the dreams of his victims into deadly nightmares.

VOA: standard.2010.05.04