During World War II resources were scarce. So my german ancestors used so called “hayboxes”.
The basic idea was this: You heat your food and then you put it into one of these boxes. The food will stay hot for hours and finish cooking inside of the box.
Picture of Haybox: Dr. Mirko Junge License: CC BY-SA 3.0
A wonderbag uses the exact same principle.
Ours was made by a pillow manufacturer here in Germany.
We use it every other day to cook potatoes and rice.
Before going to bed, we put the potatoes on to the oven until the water starts to boil. Them the pot wanders into the wonderbag and we go to bed.
There are a lot of other recipes, that just taste amazing out of a wonderbag. For example cooked wonderbag chili con carne is just amazing.
This is a great item. We save a lot of energy using the bag - and we save time as well. Win - win.