If you ever have the opportunity to play with an electric train set, be sure to smell the engine car after it has been operating. You will note a slight “electric smell.” This is the smell of ozone gas, which is created as the oxygen in the air is zapped with electrical sparks. Why is this smell sometimes apparent during a lightning storm or when you pull your fuzzy sweater off in the dry winter? Is the formation of ozone from oxygen an endothermic or exothermic reaction?