In some cases, a better approach would be to start answering the question with a question: “Why do you ask?” or "Why are you asking that question?" (This may be a bit too blunt or direct and make the customer defensive, so a better statement may be along the lines of “Hmm….that’s an interesting question. Why are you asking that?”. Understanding the reasons that the customer is asking their question helps you formulate a more appropriate reply.
For example, customers often ask whether a product has a particular capability or feature. By asking why they were asking, they might reply that they were just curious or that a competitor did or did not have that capability. Often, the customer will provide you background information on their problem(s) that this perceived capability may solve. In this case, you may not have the particular feature that they were inquiring about but, by understanding the problem, you can offer a better or alternate solution.
Let’s use a non-business (and extreme) scenario. Suppose that a married man had a girlfriend on the side and his wife suddenly asked him the question, “Who is Angela?” By first asking “Hmm. Why are you asking that question?”, the man can better assess the situation and formulate his response based on the reply.
If his wife said:
1) “Some lady named “Angela” called and asked to speak with you. She hung up when I said that you were not home.”
His reply may then be “I don’t know anyone named Angela. It must be a telemarketing call with my name on the list.”
However, if his wife said:
2) “You were asleep kissing your pillow and mumbling the name “Angela” last night.”
His reply might change to “Oh! Angela was the name of my pet turtle when I was a kid.”…or he may realize the gig is up and start packing!
Hopefully, one will always be faithful and truthful in life. It was just a good example showing the power of “Why are you asking that?”
(I haven't thought out what would happen if someone replied "Why are YOU asking ME why I am asking that?")