Science Puzzle
Combustion Dependency
You light a candle and then place a glass jar over it. The flame shrinks and goes out within a few seconds, even though the candle wax is barely used.
The wax (the fuel) is still there. The candle is not cold. Why has the flame gone out?
The Answer
The oxygen inside the jar is used up. Combustion is a chemical reaction between a fuel and an oxidiser, in this case the oxygen in air. The reaction releases heat and light as it converts fuel and oxygen into carbon dioxide and water vapour.
When the jar is placed over the candle, it traps a fixed volume of air. The candle continues to burn and consume the oxygen until the concentration drops below the level needed to sustain the reaction. The combustion reaction then stops, not because the wax has gone but because the oxidiser is exhausted.
The jar also fills with carbon dioxide and water vapour, which do not support combustion. Even if you could replace the oxygen, the presence of these products would need to be flushed out.
This is why fires in enclosed spaces are so dangerous: they consume oxygen rapidly, and both the fire and any occupants compete for the same finite supply.
The principle: Combustion requires an oxidiser. Fire is a rapid oxidation reaction. When the oxygen in a closed space is depleted below the threshold concentration, the reaction cannot continue regardless of how much fuel remains.