One of the classic symbols of winter is the igloo. Because Santa lives in the Arctic, you will sometimes see igloos in the Santa story as well. And igloos show up in cartoons all the time.

But all of these portrayals hide the fact that the igloo has been a real dwelling for Inuit Eskimos for centuries. Igloos are also a technological marvel, allowing the Inuit to build a temporary shelter or a long-lasting home out of nothing but the snow on hand. In an hour or two, a pair of Inuit Eskimos can build a warm hut from scratch. All they need is a snow knife made of bone, ivory or metal. In a day or two they can build a substantial structure that will last through the winter.

To build an igloo, you start with the right kind of snow. It needs to be hard-packed and solid. It needs to be several feet deep.

You trace out a circle in the snow that will mark the base of the igloo. A typical igloo might be 12 feet in diameter. Then, in the center of the igloo, you use your snow knife to cut out blocks of snow. A block might be 18 inches high, three feet long and 6 inches thick.

By placing these blocks on edge in a ring, they will support each other. The top of this first row is then sloped, so that the rest of the blocks will go on as a continuous spiral. The spiral shape is the key to the igloo's strength.

As blocks are added to the spiral, they tilt more and more. At the base, the blocks stand nearly upright. Eventually, at the top of the igloo, the blocks will be horizontal. To handle the changing orientation, the builder trims the edges of the blocks to the proper angles so they fit perfectly.

Once the last block is put into place at the top of the igloo, the dome takes its final form. Domes like this are incredibly strong. An igloo is strong enough that a person can stand on top of it.

Since igloos are built from the inside, the builders are now trapped in their perfect dome. With a snow knife, they can cut a door to get in and out. If desired, it is easy to add an arched tunnel around the door of the igloo as an entrance tunnel. Either way, the door hole is sealed at night with a snow block or a piece of seal skin.

By rubbing loose snow into any cracks on the outside, the igloo becomes completely sealed. Because snow contains so much trapped air, snow is a great insulator. A sealed igloo will warm up when people are inside. But the fact that it is sealed also means it is possible to suffocate inside an igloo. A hole near the top of the igloo, along with another one at the door, lets fresh air circulate.

If you were to crawl into this newly built igloo, you would notice that it is comfortable. Even though it might be minus 40 degrees F outside with a howling wind, body heat is enough to warm the inside of an igloo. You would also notice a trench in the middle of the igloo. This is where the blocks to snow to build the igloo came from.

On either side of the trench are flat areas, like shelves, at the original "ground level" of the snow. You sleep on the flat areas. Covering them with animal skins provides insulation from the snow. Animal skin blankets provide warmth while sleeping.

The trench-and-shelf structure inside the igloo helps keep the igloo comfortable. The warm air rises, while cold air sinks down into the trench. The Inuit may also use oil lamps to provide extra heat and light. The oil comes from whales or seals.

The classic dome igloo made of snow is the type of dwelling used up near the arctic circle. Other Eskimos living near forests would make houses out of wood and branches. Animal bones covered with skin is another form. Snow insulation is common in all of these structures because snow is freely available and it is very good at keeping in warmth.

The next time you see a playful drawing of an igloo, think about the igloos that people actually use as dwellings. They represent an amazing level of ingenuity, and have been essential to survival in an inhospitable landscape for many generations.

MARSHALL BRAIN is the founder of How Stuff Works.