Biofuels come from the products of living organisms, such as methane biogas from decaying manure and sewage. Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil.
Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel. For use in cars and other vehicles it is usually mixed with petrol.
Ethanol can be made by a process called fermentation. This converts sugar from sugar cane or sugar beet into ethanol and carbon dioxide. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen:
C6H12O6 2C2H5OH + 2CO2
Biofuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis.
You may wish to view this BBC News item from 2005 about ethanol as an environmentally friendly fuel.
This helps to reduce global warming (see next section).
However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.