A hydrocarbon is a chemical compound made entirely of hydrogen and carbon atoms bonded together. These molecules form the basis of fossil fuels like coal, oil, and natural gas. When you see a car driving down the street or a plane flying overhead, they're burning hydrocarbons to release energy.

The simplest hydrocarbon is methane, which has one carbon atom surrounded by four hydrogen atoms. Longer chains of carbon atoms bonded together, with hydrogen atoms attached to them, create different hydrocarbons with different properties. Propane, the fuel used in camping stoves and backyard grills, is a hydrocarbon with three carbon atoms. Octane, a key component of gasoline, has eight.

Hydrocarbons formed over millions of years from ancient plants and animals buried deep underground. Heat and pressure transformed their remains into the coal, oil, and natural gas we extract today. These fuels powered the Industrial Revolution and still provide most of the world's energy, though they also produce carbon dioxide when burned.

Not all hydrocarbons come from underground. Chemists can create synthetic hydrocarbons in laboratories, and living things naturally produce them too. The waxy coating on an apple? That's made of hydrocarbons. Even the smell of a pine tree comes from hydrocarbons that the tree releases into the air.