Over the centuries, the Japanese have enjoyed slipping into bed on cold winter nights with a simple warming device called a yutanpo. Today, we have electric blankets and bedroom heating systems, but many people still like the yutanpo—it's a gentle way to stay warm, and environmentally friendly as well.

The yutanpo is a snap to use. Take enough heated water (60 to 70 degrees Celsius) to fill a yutanpo about two-thirds full. Wrap it in a thick cloth so it won't come in contact with the skin, then put it between the sheets. You'll be warm and cozy all night, and the water will still be warm in the morning, ready to use to wash your face or the breakfast dishes.

The yutanpo was developed in China and came to Japan in the 14th or 15th century. It was in common use in Japan by the early 1700s, at the latest. In those days, most yutanpo were earthenware and had a dome shape. Kilns throughout Japan made them by the ton, in places like Shigaraki (in present-day Shiga Prefecture).

Today, when the Japanese hear the word yutanpo, they think of the oval variety made of zinc-plated iron. This type was popular in the first half of the 20th century and is still quite widely used. The top and bottom are rippled to let the heat radiate out well, and to prevent distortion during expansion and contraction. In the 1950s more than a million were made each year, but they have been largely replaced by electric heating devices. These days, production is down to about one-tenth what it was in the heyday of the yutanpo.

Because it's such a simple device, it can be surprisingly useful. It is sometimes used for relief efforts after a disaster, to keep victims warm when they have no electricity or gas. Yutanpo were sent to people shivering in the cold after the Great Hanshin-Awaji Earthquake in Japan in 1995, and after disasters in Turkey, China and elsewhere. The victims were certainly glad to have them—the warmth of a yutanpo gives a feeling of comfort to anyone who uses it.