These are the three words that create instant credibility.
"Admitting ignorance makes everything else you say more credible. Admitting ignorance marks you as a person who's not afraid to speak the truth, even when that truth might reflect poorly on you.
Needless to say, the "I don't know" should be followed by a plan to discover the information that's required, if the issue is truly important. And you WILL be judged on whether you deliver on that promise."
What do you not know? via Inc.