Turn it into a learning opportunity for the student. (and yourself)
Ya got me there! Let's find the answer together. Where would be a good place to start looking?
Trying to hide the fact that you don't know the answer hardly ever works anyway. Guiding the student(s) through the process of finding answers does them at least as much good as the answers themselves. If it's something you really should have known and included in the lesson, get them an answer as soon as you can and include it in your lesson next time.