“teach grandmother to suck eggs”别理解成“教姥姥吮吸鸡蛋”，真实含义班门弄斧。

【例句】
These young hotshots come in and try to prove they know the ropes better than we do, like they're teaching their grandmothers how to suck eggs.
这些年轻的热心人进来后，试图证明他们比我们更懂行，好像他们在班门弄斧。

Don't suggest showing Mary how to knit. It will be like teaching your grandmother to suck eggs.
不要建议向玛丽展示如何编织。这将会像是你在班门弄斧。