Whether you’re asked about the circumference of the Earth, the number of cups in a gallon, or the land speed velocity of an unladen Swallow – answering, “I don’t know,” is wrong 100% of the time.
While I would never advocate making up an answer in the hopes of your inquisitor not knowing the answer themselves, It’s never okay to say only “I don’t know,” and leave it at that. It is important to admit your knowledge gaps, but to not seek to fill them in is selling yourself short.
When you reach a mythical ‘Bridge of Death,’ which you must cross, the Keeper of the Bridge requires you to answer three questions. Telling this guy you don’t know the answer, is a really bad idea. After all he will cast you into the Gorge of Eternal Peril if you get it wrong or don’t know the answer.
Take your time, ask clarifying questions if need be, but if you really have no idea, the correct answer is “I’ll find out.”
Nearly every interview I’ve been on has included a question or two that I flat-out didn’t know the answer to. Each time, I made sure to remember or write down the question that had stumped me and to research the answer afterwards. With a little time and concerted effort, you can find the answer, and it’s a great opportunity to follow-up.
Ultimately, the answer to anything you don’t presently know is, “I don’t know … yet.”