Sometimes a fourth R is added to the three basic ones, generally standing for either "rethink" or "recover." Rethink is sometimes added to the front of the hierarchy, meaning that we should consider our options and think about their impact on the environment. Recover, which is the last R, refers to the practice of putting waste products to use. For example, decomposing garbage produces methane gas (one of the greenhouse gases), which some landfill sites recover and burn for energy rather than letting it dissipate.