Can you tell it's winter yet? Even if there's no snow on the ground, the air has probably turned cooler. It's also probably gotten much drier. Even when it's wet outside, the forced-air heating systems we use indoors crank out extremely dry air that sucks the moisture right out of the skin.

One of the tell-tale signs of winter is the dry, itchy skin that starts to irritate you shortly after the moisture level in the air drops. Not only can the itching drive you mad, but the flaky, cracked skin that comes along with it can be quite painful.

So what do you do when your skin dries out? If you're like most people, you reach for a big bottle of skin lotion. Lotion comes in hundreds, if not thousands, of varieties, but most of them have the same goal: moisturizing your dry skin.

Your skin is your body's largest organ. Even though it can burn in the summer and dry out in the winter, it's an important and tough barrier that prevents all sorts of things from getting inside your body.

Scientifically, skin is considered dry when its moisture content drops below 10%. When it gets dry, flaky, and cracked, its condition is technically known as transepidermal water loss (TEWL).

That's why it's important to keep it in good shape. In the winter, that usually means applying moisturizing lotion to keep it soft and moist.

Although lotion might seem like a relatively-new invention, people have had skin — and problems with that skin — for as long as people have lived on Earth. It's only natural that even prehistoric peoples would've looked for solutions to dry skin.

Scientists believe that Mesolithic civilizations from 10,000 B.C. may have made homemade lotion from the oil of castor plants. Archeologists have also uncovered evidence that the ancient Egyptians made homemade lotions from ingredients such as animal fat, olive oil, and spices.

Around the 15^th century, people began to make lotions that smelled good by including ingredients like beeswax, rose water, and frankincense. It wasn't until the late 19^th century that lotions began to be manufactured commercially with products such as mineral oil, petroleum jelly, and lanolin.

Today, moisturizing lotions come in a dizzying number of varieties touting all sorts of special ingredients with amazing benefits. Unfortunately, there's very little science to back up most of these claims or tell you which lotion to use.

On the bright side, however, scientists will tell you that most moisturizers do work. In fact, they basically all work in the same way by adding a bit of water to the skin along with coating substances to hold it in.

Some of those coating substances are known as occlusives, because they help hold water in by blocking it from evaporating. Popular occlusives include petrolatum, cetyl alcohol, lanolin, dimethicone, cyclomethicone, lecithin, mineral oil, stearic acid, and paraffin.

Other moisturizers may contain substances called humectants, which work by attracting water. Common humectants include glycerin, panthenol, vitamin B, sorbitol, and honey.

When the air is especially dry, however, humectants can make skin even drier by pulling water out of the lower levels of the skin rather than the air. To combat this effect, moisturizers with humectants almost always contain occlusives to hold moisture in the skin.

Moisturizing lotions might also contain a variety of chemicals known as emollients. Emollients tend to sink deeper into the skin and make it feel smooth. Some ingredients work as both emollients and either occlusive or humectants.

So which moisturizing lotion should you use? Experts recommend trying a variety of products and simply sticking with one that you like. How well a particular lotion works with your skin and whether you like its smell and feel are subjective judgments that vary from one person to another. When you find a lotion that you like that works for you, stick with it!