Renewable energy sources like biomass, wind, water and geothermal are always available and will not be depleted over time. Non-renewable sources such as fossil fuels and nuclear will eventually be used up if not replenished. Fossil fuels are burnt to produce steam that turns turbines and generators to produce electricity, while nuclear energy harnesses heat from splitting atoms for the same process. Biomass and geothermal also produce heat energy, and wind and hydroelectric use the motion of their respective resources - wind and water - to power generators.