Human: What's the difference between natural gas and propane? Assistant: 
The main difference between natural gas and propane is that natural gas is a fossil fuel, while propane is a petroleum-based product. Natural gas is made up of methane molecules, which are lighter than air and can be extracted from deep underground reservoirs or produced from renewable sources such as landfills, sewage treatment plants, and biomass waste. Propane is derived from crude oil refining processes.