Back in 2003, I gave Joe a fond farewell at his retirement party. We talked about his travel plans and his well-planned retirement investments. I knew that Joe had invested wisely, and that he was careful with his money, so I was later surprised to see him back at work.

"It was an opportunity I couldn't resist," Joe explained. "The superintendent called and asked if I'd be interested in helping oversee some of the equipment repairs for about a month."

As an independent contractor, Joe remained blissfully free of office politics. And as a part-time worker, he'd only be on the job a month. After that, he was still headed to Yellowstone.

While several younger employees might have supervised the same repairs, Joe had more experience with the unit in question than any of them, and none of the new guys had seen such an expensive outage. As part of the repair process, Joe would help train his successors. "I'm finally getting to do the fun part of the job," he told me, "without the headaches."

Getting back to work
I've known many people with stories like Joe's. When I was younger, I used to think that these individuals simply hadn't planned and budgeted well, forcing them back to work to make ends meet. But I eventually discovered that in many cases, retired employees are a boon to their former employers. Let's look at the advantages for employers and retirees alike.

We've been there, done that
In many cases, retirees possess knowledge gained from many years of experience. Employers invest plenty of money in training employees, and even retirees have a lot of skills and knowledge to pass on to their successors. Besides, we old guys enjoy showing the newcomers how smart we were!

We're already bought and paid for
Employee benefits can add 50% or more to the salary costs that companies pay for workers. But in most cases, part-time employees aren't eligible for benefits; employers can bring on a retiree for the same wage, or an even higher one, and still save money. It's therefore not surprising that companies like IBM (NYSE:IBM), Procter & Gamble (NYSE:PG), and Eli Lilly (NYSE:LLY) actively seek out retirees who want to come back to work on a flexible basis.

There's no long-term commitment
Employers may often need only a temporary employee. Most profitable companies are operating "lean and mean," lacking the staff to handle anything but routine duties. Anything out of the ordinary may require extra personnel, but only for a short time. It's a perfect opportunity for a retiree looking for a short stint back in the saddle.

Firsthand experience
As it turns out, I'm writing this article from personal experience. I retired from my job as a supervisor at Total Petrochemicals in June 2006. I truly loved my job, I enjoyed my co-workers, and I gained a lot of knowledge and experience in my particular field. However, a long and happy retirement was one of the goals for which I'd worked, so I welcomed retirement with open arms.

Last November, I was invited back to help with an extensive maintenance outage in two of the units I'd supervised over the years. My job would be to help develop a shutdown plan, then serve as an advisor during the actual maintenance outage. They wanted me to do the part of the job that I truly enjoyed, without the personnel issues that normally come with supervision! I'd be working less than three months -- a perfect length for me. The shutdown was during our cold, rainy winter, so it didn't interfere with my summer plans. I loved it.

Later, another opportunity presented itself. The refinery operating procedures and manuals need constant updating, so I was offered a chance to work part-time revising operating procedures. I'll be working about 10 days a month, mostly from my home, and with flexible hours. I must be dreaming! As much as I enjoy writing, the company's willing to pay me a great deal of money for it!

Icing on the cake
To a retiree, this is just like "found money." If we've planned like sensible Fools should, we don't need the money, or we wouldn't have retired in the first place. However, there's no such thing as having too much cash.

Few things make you feel quite as needed or appreciated as a former employer asking you to come back. If working part-time in retirement appeals to you, assess your skills. You probably acquired skills or knowledge during your career that would be valuable to your former employer, or even to someone else in the same type of business. If you think your skills are marketable, talk to your former employer. You may talk to a personnel search firm or a "temp" agency if you're really serious. These people are in business to match employer needs with employee skills. It's possible that somebody out there really wants what you have to offer.

Retirement doesn't just mean not working. It's an opportunity to enjoy yourself. And if you enjoy earning extra money and self-esteem, I'd say you're a pretty wise Fool.

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Fool contributor Glen Kenney is enjoying his retirement. He doesn't own shares of the companies mentioned in this article. Eli Lilly and Total Petrochemicals' parent firm Total SA are Income Investor recommendations. The Motley Fool has a disclosure policy.