Energy is a key resource. We burn fuels to get energy. Sometimes it is also done by the environment itself.
Fuels
as far as i know its because some fuels are atomically more dense than others, also because the technology we have may not burn some fuels as efficiently as others
We burn them
Energy led human to burn more fossil fuels
When chemicals burn and it sometimes release ashes and the fuel is converted to heat and light energy
Burn it
Fossil fuels contain the chemical energy that plants and algae obtained from sunlight and then stored in sugars. Just as energy chemical energy is stored in sugars, chemical energy is stored in fossil fuels. All it takes to release it is enough heat and oxygen to cause the fuel to burn.
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To obtain electrical energy or heat
When you burn it, the chemical energy is converted to thermal energy.
Heat and light.