In the recent thread on the use of solar energy a question was asked about the use of solar energy in other countrys.
What is the most common used fuel in the world? And just for fun the most uncommon?
I assume coal/oil/gas are the most widely used fuels, although I imagine that almost all fuels can be traced back to solar power ultimately. The rarest form of fuel used for domestic use is the piezo-electric device. You’ve got to click that little bastard a lot of times to get a light bulb to power up.
Iceland, being on the Mid-Atlantic Ridge, gets quite a bit of heating energy from geothermal.
Saudi Arabia’s main heat source is solar. 
And in Canada, I hear they either burn wood or McDonald’s litter that blows over the Great Lakes from the US.
Wood is the most common heating (and cooking) fuel used in the entire world. Deforestation is also one of the leading causes of soil erosion, climate shift and habitat destruction. For a good lesson in the importance of keeping trees around see Nepal and their mudslides.
I think the most unusual answer comes from medieval Iceland. Since there aren’t any forests in Iceland, firewood was too precious a commodity to burn just to heat the home. The Viking settlers thus built their houses partially under ground, and turfed over their roofs with sod. Then they let their livestock occasionally graze on the roof.
So the answer: cows.