I always have a "we're all learning together" motto in my classroom. When a student has a question that I cannot answer, I can use that as a real 'life-long learning moment' to show the students how to find the answer, possibly on a search engine or database, definitely illustrating the point that regardless of diploma, you never stop having to learn, teach yourself, or fact find.
Since most students all have smart phones now, it definitely wouldn't be wise to try to bluff your way out of the situation with a made-up answer! It is far too easy for students to look up the facts for themselves--which on my 'open technology campus' is exactly what I might challenge the students to do-- "Okay, class, I admit that I have no idea what the answer to the question is, so everyone pull out their phones. First one to find the correct answer is the winner!"