What if you don’t know the answer?

It’s impossible to know everything in this field, but people outside the field tend to assume that a person who works in one area of IT is supposed to know EVERYTHING there is to know about computers. The same people wouldn’t expect a sinus doctor to do back surgery.

Anyway, sometimes I’m faced with a question that don’t know what the answer is. How do you respond to those questions without looking like an idiot?