I know you knocked your knee while kneading the dough.

我知道你在揉面团时敲了一下膝盖。

He drew his wand so rapidly that Harry barely saw it; with a casual flick, the sofa zoomed forward and knocked the knees out from under all three of the Dursleys so that they collapsed upon it in a heap.

他忽地拔出魔杖，快得哈利都没看清。魔杖轻轻一挥，沙发嗖地冲了过去，撞在德思礼家三个人的膝盖上。他们一下子没有站住脚，全都栽倒在沙发上，滚作一团。