After 6 years here, I finally bought one. Actually, I didn't know about them until recently. Yutanpo is the Japanese version of a hot water bottle. They are made of totan (tin), yes, like the totan roofs some of our houses have in Hawaii. There are plastic yutanpo too, but I think the metal ones hold the heat longer.
This one is about 12 inches long and 10 inches wide and holds about 3 liters of water. Heat water to just before boiling. Add it carefully to the yutanpo, cork it tightly and slide it into the fabric bag.
Be careful! If the water is too hot, you could get mild burns.
It is best to put the yutanpo into your bed to warm it up, then when you are ready to sleep, take it out. This is to prevent burning yourself while sleeping.
In the morning, the water should still be warm, and you can use it to wash your face or dishes.
I'm hugging mine as I type.