Margolin advises saving your time and theirs by answering plainly that you don't have that answer but have a plan to learn about that area. That's OK as far as it goes, but you need a bit more.
Use discernment about how hard to sell yourself. Are they asking you about required, desired, or non-advertised skills? Time pressure might force them to choose between candidates who meet 80% of the requirements, so your can-do answer may give you the edge.
A better answer...
- admits that you don't have that knowledge
- shows that you are interested in the topic
- relates how you previously delivered value in a new area.
"I haven't worked in [topic], but I enjoy reading about [related issues] and would LOVE to get my hands dirty in it. One time, my director needed someone to deal with [relevant topic], which none of the departments covered. My supervisor asked me if I could help, so I hit the internet and the library that night, and came back with a tutorial the next day that the director liked so much, he expanded our department's charter; and naturally, my boss put me in charge of it. I know you'll be pleased with how I can help you."
Obviously, you can't give long-winded answers like this if they ask about multiple knowledge areas you don't have.