Natural resources can be classified as either renewable or non-renewable. Renewable resources like sunlight, wind, water and biomass can regenerate naturally within a human lifetime. Non-renewable resources like fossil fuels form over long geological time scales and cannot regrow once depleted. Examples of non-renewables are coal, oil, natural gas and nuclear fuels which have limited supplies that will eventually be exhausted if usage is not controlled sustainably. Both renewable and non-renewable resources are important to manage carefully to support society's needs into the future.