Next time you ask someone a question and they answer with “I don’t know” try saying this………

“It’s okay that you don’t know, but if you did know what do you think it would be?”

You will be surprised by making it Okay that they don’t know and taking the pressure off, they are able to think more clearly and provide the correct answer. 

This gives both you and them more confidence in their ability. 

By the way if the answer is completely wrong then you will need to invest some more time in either coaching, mentoring or training them. These are all topics for another day