Could it be oxygen? Fire needs oxygen to keep burning. If you have 2 candles and put a jar over 1. The covered one will burn out while the one with oxygen surrounding it stays lit.
It depends, somewhat, on what you mean by grow faster. Yes, I think oxygen is the correct answer; however, adding fuel (like gasoline) will make it grow faster, too.
Accelerant
ewbcyg
The substance that makes fire grow faster is typically an accelerant or fuel. These include materials such as gasoline, alcohol, paper, or wood. However, it's important to note that intentionally using accelerants to make a fire grow faster can be dangerous and pose a significant risk of injury or property damage. It is crucial to exercise caution and follow proper safety guidelines when dealing with fire.
