The following are three possible categories to answer the question of why you are available:

Need a Change/Challenge

Even the simple, straightforward answer can raise suspicions if the wrong message is conveyed. What if you are just tired of your job, don't like your boss, or need a change? Everyone is entitled to a new position or challenge now and then, right? Of course, but the tricky part is telling the interviewer the reason you are leaving but not sounding like you're "burned out" on your current job.

"I am looking for a new challenge. I have been with my current company for two years and don't find the work as interesting as I once did. I am looking for a company where I can take on new challenges and learn new things."

If your answer has too much emphasis on "challenge and change," the employer becomes concerned that you may be dissatisfied with this job once you've learned new things and met the challenges. The interviewer is listening for patterns, and if you were bored on your last job, what makes you think you won't get bored on this job?

Changing the tone of your reply to be more pro-active is a stronger answer.

"Since there are no advancement opportunities within the company, I decided it would be a good time for me to look outside. I have set some career goals for myself, and I know that I cannot achieve them at my current company. My goal is to work for a larger company with a possible career path."

This answer has a tone of control and planning. When you think as an interviewer, it will help you see "their" point-of-view and will address the concerns "they" have about your leaving a company.

Laid off

If you are among the millions of people who have been laid off in the last two and a half years, you can simply state, "I was laid off." This answers the question but still leaves a lingering doubt in the mind of the interviewer, - "Why were you laid off?" The more specific your answer, the more effective it will be.

"There were six rounds of layoffs at my last company. I survived five rounds, but when it came to round six they had to cut deep. My position was eliminated along with half of my group because the project we were working on was cancelled."

Not everyone will have such a definite statement to make. Whatever your situation is it will be helped by including facts and figures to explain the circumstances surrounding your layoff.

"10% of the workforce was let go," or "One out of every ten jobs was affected, company-wide."

When you quantify a statement it has more depth. When you tell the interviewer whether it was 10 or 1000 people were laid off helps put the situation in perspective.

Fired

If you were fired, you probably dread being asked this question. Not only have you been fired, you have to talk about it - over and over. How you deal with questions about being fired will depend on how you have resolved the issue with yourself.

Whether you were let go under unfair circumstances or for something you did and regret, scripting your answer ahead of the interview will help you. You don't want to bad-mouth your former employer or sound like a victim (even if you were). Practice your answer with someone in a mock interview and obtain feedback on your comfort-level while discussing your situation.

Preparing will make a difference

Any question can throw you off balance during the interview, but there are certain standard questions that you can almost expect to be asked every time. For example, "Why did you leave (or are planning to leave) your last company?" is a question that you can bet will be asked in one form or another in almost every interview. You will feel more confident and focused if you script and practice answering this question before it is asked.