Halon is a kind of gas, built from halogen and carbon, that was widely used to stop fires. It was useful in places where water would ruin machines — computer rooms, aircraft, military rooms — because it could kill a fire without soaking everything. The hard truth is that once it is in the air, it helps to damage the ozone layer, the thin shield high above us that keeps the strongest sun off the Earth. That is why many countries stopped making it and asked people to use other fire-stopping gases instead. You may still meet old halon extinguishers, or the word in news about the air and the climate. It is a technical word, not everyday chat.