kill two birds with one stone 一石二鸟

　　knock it off 别再讲下去了

　　Knock on wood. 说话禁忌;赶紧讨个吉利

　　let the cat out of the bag 泄漏秘密

　　let nature take its course 顺其自然

　　like a cat on hot bricks热锅上的蚂蚁