This is the most famous question for an interviewer and also most difficult to answer this question for the candidate. This question puts all the pressure on the candidate, and the interviewer relax.
You should alert enough to answer this question. You should start with an easy and confident tone and answer in a proper manner. It should not be scripted. Always remember, you are not giving the interview to a robot so your articulation, your pronunciation of each word should be clear and confident.