Why are fossil fuels called a non-renewable form of energy?

Difficulty: Easy

Fossil fuels are nonrenewable resources because they took millions of years to attain their present form.

We use fossil fuels such as coal, oil, and gas to heat our houses, run industry, and transport. They are usually hydrocarbons (compounds of carbon and hydrogen). When they are burnt, they combine with oxygen from the air. The carbon becomes carbon dioxide; hydrogen becomes hydrogen oxide called water; while energy is released as heat. In the case of coal:

Carbon + Oxygen      $\rightarrow$  carbon dioxide + heat energy

Hydrocarbon + Oxygen    $\rightarrow$   carbon dioxide + water + heat energy