Originally, ethylene was recognized as the active ingredient in coal gas; however, the invention of gas chromatography revealed its nature as a plant hormone. Certain stresses (e.g. drought, injury, infection, flooding) elicit a production of ethylene. Fruit ripening is also attributed to a burst of ethylene. The enzymatic breakdown of cell wall components from the ethylene gas production softens and sweetens the fruit, making it more attractive to animals. Because ethylene is a gas, it can spread through the air among fruits. This explains why storing fruits together makes them ripen faster; the ethylene builds up in enclosed conditions. This can easily be alleviated by circulating air, a reason why refrigerating fruit slows the ripening process.