Fuels are substances that release energy when burned. Most common fuels, like gasoline, natural gas, and coal, are primarily composed of carbon and hydrogen. These elements combine with oxygen during combustion to produce heat. The main products of this reaction are carbon dioxide (CO${\mathrm{CO}}_2$) and water (H$H_2$O). While some fuels may contain small amounts of nitrogen or oxygen, carbon and hydrogen are the fundamental components.

Final Answer: Carbon and Hydrogen