Here's a real-life example:
            
An interviewer asked Mary (not her real name):
            
"Why did you leave your last job?"
            
Here's what Mary doesn't want to tell them:
            
She was let go because her last employer was bringing in young talent at a cheaper price. Mary was 57 at the time and had served the corporation for many years, which meant she was considerably more expensive. So she left voluntarily so that she wouldn't be pushed out kicking and screaming.
            
Now obviously, if she tells that to an interviewer, they'll use her honesty against her, for two main reasons:
            
First, it implies (rightly or wrongly) that a younger and less experienced employee (who costs less to hire) can do her job to a satisfactory level.
            
(Otherwise, from their point of view, they'd expect that Mary's employer would have fought to hold onto her. Something must be wrong.)
            
Second, Mary would be openly accusing her previous employer of ageist discrimination. (Even if it's true, you never want to badmouth a past employer in your interview. It always works against you.)
            
On the other hand, Mary has tried a cookie-cutter answer like "I was looking for a new challenge", and interviewers looked at her as if she was an idiot.
            
What can she do here?
            
What could you do, if you were to face a similar dilemma?
            
Here's how I would think about it:
            
Have you ever had a child ask you a tricky question, like -- "where do babies come from?" Well, when you answer their question, you simplify things a bit.
            
You don't lie to them, but you don't tell every detail either. You make the story simpler -- and you avoid any drama.
            
That's what I recommend for this type of interview question. Keep it simple enough and inoffensive enough that you'd feel comfortable sharing it with a young child. So, in the case of Mary's situation above, she could say:
            
"What I enjoyed most about my job at ABC company was doing [she should mention where she did her best work]. A few months ago, my department went through a reorganization, and, as a result, my role and responsibilities changed. I decided to leave so that I could focus on landing a position where I deliver my best each day. I am excited to be here today because, based on everything I've learned so far, this position seems to be a strong fit."
            
That's the truth.
            
It's not the entire truth. It might not even be the "main" truth. But it's all that the interviewer needs to hear. It conveys maturity, clarity, and a sense that Mary can provide value.
            
That's how you answer "impossible" questions like this.
            
However, be warned -- an answer like this gets you out of a tight spot, but it won't win you the interview. You need to ace all of the other questions too.