Fuel is any material that stores energy which can be released to do useful work. When you burn wood in a fireplace, the wood is fuel: it releases heat and light energy that was stored in it. Gasoline fuels cars, natural gas fuels stoves, and coal once fueled steam engines and still fuels many power plants.

Your own body runs on fuel too. The food you eat, especially things like bread, rice, and potatoes, provides the energy your muscles and brain need to function. Athletes talk about “fueling up” before a big game because they need that stored energy.

The word also works as a verb. You fuel a car by pumping gasoline into its tank. More figuratively, one thing can fuel another by making it stronger or more intense. Curiosity fuels scientific discovery. A rivalry between two teams might fuel an exciting season. Encouragement from a teacher can fuel a student's confidence.

Different fuels work better for different purposes. Rocket fuel needs to pack enormous energy into a small space. Diesel fuel works well for heavy trucks. People often say your brain runs best on steady fuel from healthy meals throughout the day, not just candy bars that burn up quickly and leave you tired afterward.