Don't be thought a fool. Open your mouth and prove it!
I once had a manager who used to love to set people up by telling them to explain things to me slowly and in words of few syllables as I was not the sharpest knife in the drawer.

@FrayedBear A favourite saying of mine 'Better to remain silent and risk appearing the fool than open your mouth and remove all doubt.'

@Surfpirate Exactly, I just turned it around in response to being reminded by @Rugglesby of a manager with a great sense of humour who liked to knock tall poppies down.

@FrayedBear I haven't heard the term tall poppies in ages. thanks

@Surfpirate you are welcome.