When the heat comes on in October, your skin feels it first. The backs of your hands go tight. Your shins itch under your socks. A fine white dust shows up when you pull off dark pants.

So you buy a lotion. Then a richer one. Some jars cost $20 or $30 and promise to "lock in" moisture. And for a few hours, they help.

But the lock on your skin isn't made in a jar. Your body builds it from the inside, out of fats. One of those fats your body can't make at all. You have to eat it. And one of the cheapest places to find it is a bag of plain sunflower seeds...

We know this because of a husband, a wife, and some very sick rats.

In 1924, a young chemist named George Burr went to work at the University of California, Berkeley. His lab fed rats food scrubbed of nearly every trace of fat. The plan was to study a vitamin. Instead, the rats fell ill in a way nobody could explain.

Then Burr took a job at the University of Minnesota. In the fall of 1928, he and his wife, Mildred, loaded two cages of rats into a Model T Ford and drove east. On cold nights, as he told it years later, "our pets were smuggled into hotel rooms under long overcoats."

Mildred had run the rat colony back in Berkeley. In Minnesota, money was tight, so she worked beside him for free. Together they cut fat from the rats' food more strictly than anyone had tried before.

Take a look at what happened. After a few months, the rats' skin turned scaly. Their tails swelled and grew ridges of scales. Dandruff filled their fur. They lost more water through their skin than healthy rats did. Left alone, they died.

But three drops of lard brought them back.

A year later, the Burrs named the part of the fat that mattered. It was linoleic acid. They called it an "essential" fat, a name still in use. Many nutrition experts doubted it. Burr later recalled that someone sent him a letter of condolence for his idea...

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Let me explain what those rats were missing.

The outer layer of your skin works like a brick wall. Its bricks are flat, dead skin cells. Fat fills the gaps between them, like mortar.

And the Burrs' fat is one of that mortar's building blocks. Your body hooks it into a special waxy fat, then bolts that fat onto the bricks. That's the seal. It keeps water in and dry air out. Without it, the mortar crumbles. Water slips out through the gaps, and the skin flakes, just like those rats' skin did.

You see, the same thing turned up in people. In the 1950s, a children's doctor named Arild Hansen, who had once studied under Burr, watched babies fed a formula with almost no fat. Of 24 babies kept on it for at least a month, 15 grew dry, thick, flaky skin. When his team added linoleic acid back in, the skin cleared.

Today, the rules for baby formula require this fat.

What 4,025 women showed: In a large U.S. health survey, skin doctors looked over women aged 40 to 74 in person. The women who ate more of this fat had about 25% lower odds of the dryness that comes with age, and about 22% lower odds of thin skin.

Of course, you're not a baby on a fat-free formula. A true shortage of this fat is rare in grown-ups, since most of us eat some every day. So does eating more of it matter for skin like yours?

A research team at Unilever went looking. They used a federal survey from the early 1970s, where skin doctors had checked each woman's skin up close. Each woman also told the survey what she had eaten. Those results came out in the American Journal of Clinical Nutrition in 2007.

The women with more of the Burrs' fat on their plates were less likely to have the dry, thin skin of later life. That held after the team accounted for age, sun time, weight, income, menopause, and supplement use.

And here's the twist. More fat overall went the other way. It tracked with more wrinkles. So the answer isn't more fat. It's the right kind.

Now for the limits. This was a snapshot, not a trial, so it shows a link and not proof. The food came from what women recalled. Unilever sells skin creams, so it had a stake in the topic. And I couldn't find a single trial that handed older adults sunflower seeds and then measured their skin.

Still, the pieces line up: rats in 1929, babies in the 1950s, and thousands of women in a survey. A cheap seed fits right into that picture.

The sunflower is an American plant. People in North America were growing it for food thousands of years before ships from Europe arrived.

The dried kernel is packed with the Burrs' fat. By USDA figures, a one-ounce handful carries about half or more of what an adult over 50 is advised to eat in a day. It's also one of the richest common foods for vitamin E. And it's familiar, cheap, and keeps for months in the cold, just what an everyday food should be.

You may have heard seed oils called bad for you. That fight is mostly about bottled oils and fried food, and I won't settle it here. A handful of whole seeds is a different thing. You get the fat with fiber, protein, and crunch, and nothing gets fried.

Look for raw or dry-roasted kernels with no salt. Flavored and salted seeds can pack a lot of salt into a small bag. This kind of fat goes stale fast, so keep the bag in the fridge. If the seeds smell like old paint or crayons, toss them.

At the University of Sheffield, 19 adults rubbed six drops of oil on their forearms twice a day for four weeks. Sunflower seed oil kept the skin's wall strong and helped it hold water. Olive oil did the opposite, weakening the wall and leaving mild redness. Sunflower oils differ, though. The ones bred for frying, labeled "high oleic," carry far less of the skin's fat.

Long, hot showers strip fat from your skin. Keep them short and warm. Pat dry, then smooth on cream or oil while your skin is still damp.

The Burrs smuggled sick rats into hotel rooms to prove a point most experts brushed off. A few drops of the right fat put that skin back together.

Your version is easier. It's a jar of seeds on the fridge shelf.

It won't change the weather, and it won't replace your doctor's advice. But it hands your skin the one fat it can't make on its own. Buy one bag of plain sunflower seeds this weekend. Eat a handful a day. Then look at the backs of your hands on the first of November.