I have told people this is too complicated for me to explain ... because I don't understand it well enough to explain it. My wife doesn't appreciate when I explain in too much detail when I do know it so I try to adjust explanations to the person's background and temperament. I'm not always successful as sometimes my sarcasm gets the best of me and I say the perfect thing at exactly the wrong time.
I do try to supply some answer when I have an idea, a guess when I can make a guess (making it clear that it's a guess) and I've started saying that I have no idea when I have no idea.
The most important thing (especially for our museum docents) is that we always * try * to explain and never look down on guests. Hopefully we get it mostly accurate, too.