Dead animals and plants rot and decay.
In swampy areas, for example, dead plants fall into the water. As they decay, they give off gas. Scientists call this biogas.
Biogas comes from garbage, sewage, and manure, too.

Biogas is a source of cheap energy for many villages in China. Many vehicles are fueled by methane, a mixture of biogas and carbon dioxide.
Biogas is a valuable source of energy which is often wasted. When materials decay, the biogas mixes with carbon dioxide, forming a gas called methane. Large amounts of methane can be dangerous—a tiny spark near the gas can cause an explosion. But if methane is collected safely and stored correctly, it can provide an important source of energy.

Biogas could be important in countries where other fuels are scarce or too expensive. A small village can use its own garbage and sewage to produce biogas. People collect the droppings of farm animals and put them in a tank. Some of the waste turns to liquid. The liquid soon starts to give off methane, which is kept in a storage tank that has an expandable lid. The gas can be used in homes for cooking and heating, or as fuel to power an engine or an electric generator.