You can't feel your bones grow weak. That is the scary part.

Right now, you feel fine. You stand, you walk, you carry the bags in from the car. But deep inside each bone, a slow change may be under way. The frame that holds you up can get thinner year by year. And you would never know it until the day it gives.

Here is what is at stake. Half of all women over 50 will break a bone from weak, brittle bones. Doctors call this osteoporosis — bones so thin they snap too easy. Men are not safe either. One in four men over 50 will break a bone this way. A hip can go from a small trip on a rug. And a broken hip late in life can steal your freedom for good.

So what guards those bones? You might guess a pill. Or a tall glass of milk. But one of the best foods for the job is small, sweet, and cheap. It costs about two dollars a bag. And it is the humble prune…

Let me tell you about this fruit.

A prune is just a plum that has been dried. People have grown plums for thousands of years. The Romans prized them. Traders hauled them along old roads from Asia into Europe. Then the trees reached California, which now grows most of the world's prunes.

For years, the prune had an image problem. The name made people think of old age and a slow gut. So in the year 2000, growers put a new name on the label: "dried plums." It is the same fruit with a fresh coat of paint.

Your grandmother kept a box of them in the cupboard. She ate a few when things felt stuck, and she was right — the fiber and sorbitol in a prune keep you regular. But this little fruit does far more than that. And labs have now found it may guard the one part of you that you cannot see wearing down: your bones…

Here is how your bones work. They are not dead sticks. Bone is living tissue, and your body makes it new all the time.

Picture two small crews at work inside each bone. One crew breaks down old bone. The other lays down fresh bone. When you are young, the two crews stay in balance. Your bones hold strong.

But age tips that balance. After menopause, a drop in estrogen speeds up the crew that tears bone down. Now the tear-down crew outruns the builders. Bit by bit, your bones grow thin. That slow loss is what leads to weak, brittle bones.

So where does the prune fit? It is loaded with plant compounds called polyphenols. These compounds seem to calm the tear-down crew. They also cool the slow swelling that spurs that crew on. Less tear-down means you keep more of the bone you have.

And the prune brings more to the table. It carries vitamin K and boron, two things your bones need to stay firm. The potassium inside helps your body hold on to bone minerals. Six prunes even feed you a little copper and fiber. That is a lot of help from one small, sweet fruit…

Penn State's Prune Study: After one year, women who ate no prunes lost bone at the hip. Women who ate just five to six prunes a day held on to it.

For a long time, this was only a hunch. Then two teams put it to a real test.

The first worked at Florida State University. A group led by Bahram Arjmandi ran a trial for a full year. They asked women past menopause to eat about ten prunes a day. A second group ate dried apple instead. By the end, the prune eaters had stronger bone in the spine and forearm. Their blood showed less bone breaking down. The work ran in the British Journal of Nutrition in 2011.

Then a larger test came out of Penn State. Mary Jane De Souza led it and named it the Prune Study. She tracked 235 women past menopause for a year. One group ate no prunes at all. The others ate a small handful each day. That no-prune group lost bone at the hip — the very break that can end a person's freedom. But the prune eaters kept theirs. And just five or six a day did the trick. The findings ran in the American Journal of Clinical Nutrition.

Take a look at what that means. The hip break is the one doctors fear most. And a cheap, sweet fruit helped keep that bone in place…

A prune is not the whole story, of course. Your bones do best with a team around them.

Move your body, and your bones grow stronger. Weight-bearing work tells them to build — a brisk walk, a climb up the stairs, a bag of groceries in each arm. Eat calcium too, from plain yogurt, canned salmon with the soft bones, or a small handful of almonds. Get a bit of sun for vitamin D, which helps your body use that calcium. Prunes do their best work as part of that crew.

And you get a bonus. The same fiber that helps your bones keeps your gut moving well. So the fruit your grandmother trusted for one job quietly pulls off two…

Five or six prunes a day is the amount the Penn State work used. Build up to it slowly. Too many at once can send you running to the bathroom, thanks to the fiber and sorbitol. Your gut will settle in a week or two.

Skip the ones rolled in sugar or candied for snacking. You want plain dried plums with nothing else on the list. Soft, moist ones taste best and are easy to chew. A sealed bag keeps for months in the pantry.

You will eat more prunes if they sit in plain sight. Chop a few into your morning oats or yogurt. Stir some into a stew, or blend them into a smoothie. Keep a small bag in your desk drawer for the afternoon slump.

Your grandmother was onto something. She kept those wrinkled little plums in the cupboard for good reason. And now two labs have shown her hunch had real bone behind it.

You cannot feel your bones thinning. But you can do something about it, starting with your next trip to the store. Grab a bag of prunes. Eat five a day.

Give it a year, and your bones may thank you. Buy a bag this week. See how easy the habit feels by Friday.