When animals go poop, they make something called dung. This dung is then left to dry out in the sun or wind, making it very hard and dry. People can then use this dried dung as a fuel to make fires and heat their homes. It's kind of like using wood or coal, but it comes from animal poop instead. Of course, it's very important to make sure the dung is completely dry before using it, because wet or damp dung won't burn well and could make a mess. People have been using dried dung as fuel for a very long time, especially in places where there aren't a lot of trees or other sources of fuel. Some people even use it to cook their food!