Living things, or organic matter, contain organic molecules composed of carbon, hydrogen and oxygen which will burn in oxygen, just like petrol fuel does, to give energy. The energy is released when the carbon and hydrogen atoms are converted to carbon dioxide and water, respectively. The reason that most living things don’t burn is that they contain a lot of water which would tend to put the fire out. You need to add a good fuel, such as petrol, to make the fire hotter to drive off the water.
Wood is one of the exceptions in that it naturally doesn’t contain as much water. Even so, there are some types of wood that need to be dried out before they will burn properly.
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