Usually asked at the start of an interview, this is an opportunity to find out straight away how good you are at thinking on your feet.

The easiest way to think on your feet in this situation is simply to think ahead. It may sound obvious, but if you know the question is likely to come up, a little time rehearsing a potential answer will help you remain calm and collected.

Something short, positive and relatively non-specific e.g. I didn't find the work challenging enough, and that's what I really like about this position'. Always be prepared to give examples.