To be honest, I don't think it's a very good question. When I am interviewing someone I rarely ask it. The only reason is to catch someone out in exactly this way.
That's the point of OP's question...? If the answer were obvious, we wouldn't be talking about it.
I'm moving jobs very soon, and the reason I gave to the interviewer was that I didn't like my geographical location. I wanted to move somewhere better, more active.
[–] joseremarque ago
To be honest, I don't think it's a very good question. When I am interviewing someone I rarely ask it. The only reason is to catch someone out in exactly this way.