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It all starts with sunlight.
Sunlight pours energy on the Earth, and the energy gets converted from one form to another, in an endless cycle of life, death, and renewal. Some of the sunlight got stored underground, which has provided us with a tremendous "savings account" of energy on which we can draw. Our civilization has developed a vast thirst for this energy, as we've built billions and billions of machines large and small that all depend on fuel and electricity.
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