As apparent in its name, natural gas is the product of naturally occurring processes. All natural gas in use today began with living organisms. Living plants, animals, and microbes store energy as complex organic carbon molecules within their bodies. When these organisms die and decompose, these complex carbon molecules are broken down to eventually form the simpler organic molecules that make up oil and gas. Certain microbes even produce natural gas as a byproduct of metabolism. The activity and growth of these organisms gives rise to two main types of natural gas: thermogenic natural gas and biogenic natural gas.