Saying for "don't talk about things you don't understand"

Question

Is there a (relatively) common phrase, preferably using a metaphor, which means one should not talk about things one does not really understand? I have the impression of some such phrase floating around in my memory, but all that I can think of is Do not meddle in the affairs of wizards, which is not the phrase I am looking for.

Answer 1

I've always liked:

Better to be thought a fool than to open your mouth and remove all doubt.

And:

You are in way over your head. [Better keep your mouth shut or you'll drown.]

Answer 2

A phrase I often use...

I'm out of my depth!