Every so often, a question comes along that’s so simple it catches people completely off guard.

Years ago, when Ted Kennedy was thinking about running for president, journalist Roger Mudd asked him a pretty basic question: “Why do you want to be president?” That moment became famous — not because the question was hard, but because he didn’t have a good answer to it. It was vague and rambling and that’s what made it embarrassing. If you’re running for president, you’d think you’d have that one figured out.

Retirement has its own version of that question.

“When you think of your retirement, what does it look like?” Or maybe even more basic: “How do you define retirement?”

On the surface they seem like simple enough questions, but if you ask someone (or yourself) who’s close to retirement, you might find that the answers aren’t as simple…or as clear.

They might know when they want to retire — 65, 67, 70, whatever number they’ve landed on. They’ve got a spreadsheet: savings, Social Security, safe withdrawal rate, all of it mapped out. Ask them about the money, and there’s a good chance they’re reasonably ready.

Ask them what a week looks like six months after they retire, though, and things get vague fast.

That’s the problem. Retirement isn’t just the absence of work — it’s what replaces it. And “what replaces it” is a question a lot of people haven’t really considered. Where will you get your social fix? What will your purpose be? What are your new goals?

Maybe your answer is travel. More time with your spouse. Golf. Grandkids. Volunteering. A part-time job, or a business you’ve always wanted to start. Writing, music, learning something new. Some mix of all of it.

There’s no perfect answer here, but I do think there’s a right approach: have your answer, and have a plan.

I’m not saying you need the next 20 years mapped out day by day — that’s not realistic anyway. Your interests will shift. Your health will change. Family circumstances will change. Opportunities will show up that you can’t predict today. Nobody plans that precisely, and nobody should try to.

Think about it the way you’d think about anything else that matters. If someone asked you to sit in on a meeting, you’d want to know what you were there to accomplish. If you’re heading to an event, you usually have a reason for going. Retirement shouldn’t be any different.

So, as you nail down the finances and the timing, be sure you make room for the retirement version of Kennedy’s question: not just why you’re doing this, but what you actually want from it.