What would you do if the interviewer starts asking you technical questions that are not related to your background?

Its best to be honest than given wrong answer unless you have some knowledge on that topic.

I'll say frankly " The answer is not with me at the moment, but i can provide it if you let me open my mobile". I believe it's honest, show initiative and funny in professional manner.

I would say that  I am not too familiar with it but will get back to you later with a correct answer. Or you may say that you dont know and that if you were to guess you would think its.................. 

I ask the interviewer if this a part of my future job.... and let the person know that I have no expertise on this subject if the answer is yes.

If the answer is no I the interviewer continue with his/her questions. If the person insists on this question I will let him/her know that I would have the answer at the moment and would have  I to look up the answer  

I will be honest and say: "I don't know, it is not within my scope"

If the career back ground is examined by the interviewer and found that you are from a particular industry, they may ask such questions even though you are not a technical candidate.  Let them try with such questions to know about your involvement in the field where you have worked before. Though my career is based on finance and accounts, If my last employment is from an establishment belong to an Engineering Industry or Building industry  or automobile industry--normally I expect some general/technical  questions like:

What are different grades for the cement? and what is P53 ?

What is5 G or6 G welding and what are the positions?

You know how to read Drawings? ...........................

.................If I say no to these kind of questions...naturally I cannot expect that job.

But for the candidates who are fresh applicants and not their experience belong to such fields can definitely deliver a straight forward answer as mentioned by the colleagues here.

Just relax , tell them that you do not know.  One has to accept that in this world with a bundles and bundles of technical growth, it is very difficult to keep abreast of technology development unless that person uses that particular skill in his work profile. Also one has to remember that if you say that you do know , they must not come in to a conclusion that you do not know nothing.

I'd ask how it relates/might relate to the job profile i'm being interviewed for.

If it is something that is outside your area then - give a straightforward answer that it is not within your area of expertise and that you are not able to give an answer.

Answer these as honestly as possible. Let the interviewers know that you are not an expert in the field.