“like a knife through butter”别理解成“像刀切黄油一般”，真实含义是几乎不费吹灰之力；轻而易举；毫无困难。

【例句】
Lasers can slice through rock like a hot knife through butter.
激光可以轻而易举地切开石头。

It went through the metal door like a knife through butter.
这种东西切割金属门几乎不费吹灰之力。