You want to be a great boss. You want your company to be a great place to work. But right now, at this very moment, one of your key employees might be about to walk out the door.

She has consistently brought her best game to work and has grown into a huge asset. But her Learning has peaked, her growth has stalled, and she needs a new challenge to reinvigorate her.

As her boss, you don’t want anything to change. After all, she’s super-productive, her work is flawless, and she always delivers on time. You want to keep her right where she is.

That’s a great way to lose her forever.

This was my situation more than a decade ago. After eight years as an award-winning stock analyst at Merrill Lynch, I needed a new challenge. I’ve always liked mentoring and coaching people, so I approached a senior executive about moving to a management track. Rather than offering his support, he dismissed and discouraged me. His attitude was, We like you right where you are. I left within the year.

This kind of scenario plays out in companies every day. And the cost is enormous in terms of both time and money. But if I had stayed and disengaged, the cost may have been even higher. When people can no longer grow in their jobs, they mail it in — leading to huge gaps in productivity. According to Gallup, a lack of employee engagement “implies a stunning amount of wasted potential, given that business units in the top quartile of Gallup’s global employee engagement database are 17% more productive and 21% more profitable than those in the bottom quartile.”

And yet engagement is only symptomatic. When your employees (and maybe even you, as their manager) aren’t allowed to grow, they begin to feel that they don’t matter. They feel like a cog in a wheel, easily swapped out. If you aren’t invested in them, they won’t be invested in you, and even if they don’t walk out the door, they will mentally check out.

How do you overcome this conundrum? It starts with recognizing that every person in your company, including you, is on a learning Curve. That learning curve means that every role has a shelf life. You start a new position at the low end of the learning curve, with challenges to overcome in the early days. Moving up the steep slope of growth, you acquire competence and confidence, continuing into a place of high contribution and eventually mastery at the top of the curve.

But what comes next as the potential for growth peters out? The learning curve flattens, a plateau is reached; a precipice of disengagement and declining performance is on the near horizon. I’d estimate that four years is about the maximum learning curve for most people in most positions; if, after that, you’re still doing the exact same thing, you’re probably starting to feel a little flat.

Take my own career: I moved to New York City with a freshly minted university degree in music. I was a pianist who especially loved jazz. But I was quickly dazzled by Wall Street which, in the late 1980s, was the place to work. I secured a position as a secretary in a financial firm and started night school to learn about investing.

A few years later, my boss helped me make the leap from support staff to investment banker. It was an unlikely, thrilling new opportunity that required his sponsorship and support. After a few years, I jumped again to become a stock analyst, and I scaled that curve to achieve an Institutional Investor ranking for several successive years.

When I began, I was excited to be a secretary on Wall Street. I was also excited to become an investment banker. And I loved being a stock analyst. Though I started in each of these positions at the low end of their respective learning curves, I was able to progress and achieve mastery in all of them.

Eventually, I became a little bored with each job and started looking around for a new challenge to jump to. Most of us follow similar patterns — our brains want to be learning, and they give us feel-good feedback when we are. When we aren’t, we don’t feel so good. The human brain is designed to learn, not just during our childhood school years but throughout our life spans. When we are learning, we experience higher levels of brain activity and many feel-good brain chemicals are produced. Managers would do well to remember that.

Because every organization is a collection of people on different learning curves. You build an A team by optimizing these individual curves with a mix of people: 15% of them at the low end of the curve, just starting to learn new skills; 70% in the sweet spot of engagement; and 15% at the high end of mastery. As you manage employees all along the learning curve, requiring them to jump to a new curve when they reach the top, you will have a company full of people who are engaged.

You and every person on your team is a learning machine. You want the challenge of not knowing how to do something, learning how to do it, mastering it, and then learning something new. Instead of letting the engines of your employees sit idle, crank them: Learn, leap, and repeat.