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# Fuel

A fuel is a substance that is changed in some way to produce heat, electricity, or other forms of energy. This is usually by being burnt, although there are exceptions, such as nuclear fuel. Fuel can be divided into two main types: fossil fuels and other fuels.

## Fossil fuels

There are several kinds of fossil fuels:

Some people do not want us to use fossil fuels because they may pollute the environment. Because all fossil fuels are made out of carbon, burning them makes carbon dioxide. Some people think that too much carbon dioxide in the air may cause a greenhouse effect. Fossil fuels are also non-renewable, or renew extremely slowly (thousands of years). This means that we will eventually run out of fossil fuels.

## Other fuels

Other fuels are not made from fossils. Some of these, like nuclear and hydrogen, do not have carbon in them. Others, like wood or ethanol, do have carbon in them. Some examples of these fuels are:

These fuels are renewable. This means that we can make more of them if we run out. For instance, if we run out of ethanol, we can grow some more corn and make some more ethanol.