What actually is fire?
Fire is not a substance or an object. It is a process — a self-sustaining chemical reaction — and what you see is the visible byproduct of that reaction.
The chemistry. Fire is rapid combustion: fuel reacting with oxygen, releasing heat and light. The reaction breaks chemical bonds in the fuel and forms new, lower-energy bonds in the products, and the difference is released as energy.
Crucially, solids and liquids do not burn. What burns is gas. Heat first drives volatile gases out of the fuel — a process called pyrolysis for solids like wood. Those gases mix with oxygen and ignite. This is why wood must be heated before it catches, why a candle wick draws up liquid wax that vaporises before burning, and why blowing out a candle leaves a trail of smoke you can relight from an inch above the wick — that smoke is unburnt fuel vapour.
What the flame actually is. A region of hot gas where the reaction is occurring, containing:
Incandescent soot particles. Tiny carbon particles heated until they glow — this is the source of most of the yellow-orange light in an ordinary flame, and it is the same black-body radiation that makes hot metal glow.
Excited molecules and radicals emitting at specific wavelengths, which produces the blue at a flame's base.
Hot gases and plasma in very hot flames, though most everyday flames are not substantially ionised.
Why flames point upward. Hot gas is less dense and rises through convection, drawing fresh air in at the base. In microgravity, with no buoyancy, flames form spheres and burn dimly blue — a striking confirmation that the familiar teardrop shape is entirely a gravity effect.
The fire triangle — fuel, oxygen, heat — plus the sustaining chain reaction. Removing any one extinguishes it, which is how every extinguisher works.