How to steer the interview is to be honest and to talk.

"That's a cool problem. I'm curious though - I didn't see that on the job description. I'm not expert on that, and I didn't bone up on that last night. (ha ha everyone laughs)

I can step through it if you want, but it would help me if I knew why you asked about this. If you're looking to understand how I solve problems, that's okay, I'll muddle my way through it and ask if I need any clarifications."

They'll tell you. Interviewers often hold similar views to the candidates, but may be asking for a different reason than you think.