1. Fuel: Any substance that can burn and release energy is considered fuel. Common fuels include wood, paper, gasoline, and natural gas.

2. Oxygen: Fire needs oxygen to sustain itself. Oxygen is present in the air we breathe, but it can also be supplied from other sources, such as oxygen tanks or chemical reactions.

3. Heat: Heat is the energy required to raise the temperature of the fuel to its ignition temperature, the point at which it will start burning. Heat can come from various sources, such as a spark, a flame, or friction.

When these three elements are present and combined in the right proportions, a chemical reaction called combustion occurs, resulting in fire.