Sorry i didn’t say what it was. Yes it is a hot tub. The fuel source is what ever you want. You build a fire inside the circle of coils. As the water in the coils heats up it begins to circulate, eventually heating all of the water. It would be pretty easy to build something like this to whatever scale you wanted. With a little work you could position the coils around the smoke stack of a wood burning stove and be heating water without taking space up on the cook top. You could have a sink next to the stove for washing dishes.You might even be able to work a small water heater like this. You would have running hot water as long as you had water pressure in your pipes. I guess you could pet a grate over the coils and use it as a grill too. It just really struck me funny to see something like this in Skymall.