What is fire, exactly?

The answer

Fire is a self-sustaining chemical reaction called combustion, where fuel reacts with oxygen to produce heat, light, carbon dioxide, and water vapor. The visible flame is a mixture of hot gases and incandescent soot particles. The color indicates temperature: blue flames (at the base) are hottest at around 1,400°C, while yellow and orange flames are cooler regions where carbon particles glow. Fire is technically a plasma at its hottest points, making it a fourth state of matter in those regions.